Flank pain is discomfort in the area on the side of your body that runs from the lower ribcage down to the top of the hip. It can feel like a dull persistent ache or a sharper pain that comes and goes, sometimes radiating toward the front or into the groin. Common causes include kidney issues, muscle strain, posture problems, and digestive conditions.

When should I see a doctor for flank pain?

See a doctor promptly if your flank pain is severe or worsening, accompanied by fever or chills, associated with blood in the urine, radiates to the groin with nausea and vomiting, involves difficulty urinating, or follows a fall or direct trauma. These symptoms may indicate kidney stones, kidney infection, or other conditions requiring medical evaluation.

How can I find the cause of my flank pain?

Tracking your symptoms over two to three weeks can help identify your personal pattern. Log pain intensity and location, timing, activity levels (including the day before pain appears), hydration, digestive patterns, and stress levels. Patterns often emerge that are invisible day-to-day - for example, pain consistently appearing 24-48 hours after exercise, or worsening after long days sitting at a desk.

Flank Pain: Causes, Triggers, and How to Find Yours

Q: What are the most common causes of flank pain?

The most common causes of flank pain include muscle strain (particularly in the quadratus lumborum and lower back muscles), kidney issues such as kidney stones or urinary tract infections, digestive conditions like IBS, gas, or diverticulitis, and behavioral factors like prolonged sitting, poor sleep position, and sudden exercise intensity spikes. In most otherwise healthy people, muscle strain from posture or activity patterns is the most frequent cause.

Q: Can poor posture or sitting cause flank pain?

Yes. When your lower back isn't properly supported in a seated position, the deep flank muscles - including the quadratus lumborum - compensate for hours at a stretch. Research on sedentary behavior consistently shows that prolonged sitting time is a significant risk factor for lower back and side pain. Carrying heavy loads on one side, sleeping in a position that bends the lower spine, and sudden increases in exercise load can all contribute to muscle-related flank pain.

Flank pain - that ache or sharp sensation in the side of your torso, between your lower ribs and your hip - is one of those symptoms that can mean a lot of different things. Most people who search for it already know that kidney stones and infections are on the list. What they don't usually find is a clear answer to the more personal question: given everything it could be, which one is it for you?

This article covers the most common causes of flank pain, including the behavioral and lifestyle ones that medical sites tend to gloss over, and gives you a practical framework for narrowing it down.

What Is Flank Pain, Exactly?

The flank is the area on the side of your body that runs from the lower ribcage down to the top of the hip. It sits over a lot of important anatomy - the kidneys, which sit high in the back on each side, the large muscles of the lower back, the colon, and in women, the ovaries.

When people describe flank pain, they often mean one of two things:

A dull, persistent ache that seems to live on one or both sides
A sharper pain that comes and goes, sometimes radiating toward the front or into the groin

The character, location, and pattern of the pain are all useful clues - which is exactly why tracking what's happening when the pain occurs can be more useful than guessing from a list.

What Are the Most Common Causes of Flank Pain?

Could It Be a Kidney Issue?

The kidneys are the first thing most people think of with flank pain, and for good reason - they sit right in the flank region and they're vulnerable to a few conditions that cause pain in this area.

Kidney stones are one of the more dramatic possibilities. The pain from a kidney stone passing through the ureter tends to be severe, colicky (coming in waves), and may radiate from the flank toward the groin or lower abdomen. It's often accompanied by nausea and changes in urination.

Urinary tract infections (UTIs) that have spread to the kidneys (called pyelonephritis) can cause a duller but persistent flank ache, usually combined with fever, burning on urination, and urgency.

Dehydration is worth mentioning here too. The kidneys are highly sensitive to hydration status, and some people find that inadequate fluid intake is associated with a low-grade flank discomfort - though this is less specific than stone pain or infection symptoms.

If your flank pain is severe, comes with fever, changes in urination, or blood in the urine, a medical evaluation is urgent and should not wait.

Could It Be Muscle Strain?

This is by far the most common cause of flank pain in otherwise healthy people, and it's the one that gets the least airtime in most medical articles.

The muscles of the lower back and flank - particularly the quadratus lumborum (a deep muscle that runs between the lower ribs and the pelvis) and the erector spinae group - can become strained or develop trigger points from:

Prolonged sitting in a poorly supported position, where the lower back muscles work hard to compensate for poor alignment
Lifting or twisting movements that load the lower back asymmetrically
Exercise intensity spikes - doing significantly more activity than usual (a long hike, a gym session after weeks off) can leave these muscles sore for days
Sleeping in an awkward position, especially on a soft mattress that lets the lower spine sag overnight

Muscle-related flank pain tends to be one-sided, worsens with certain movements (particularly bending or twisting), and is often worse in the morning or after periods of sustained inactivity. It generally improves with gentle movement and heat.

Could Your Posture or Activity Patterns Be Driving It?

This is the behavioral angle that most people miss entirely. The way you move, sit, sleep, and exercise throughout the week creates cumulative loads on the muscles and structures of the flank region. For many people, their flank pain isn't caused by a single incident - it builds gradually from a combination of daily habits.

A few patterns worth looking at:

Desk posture and chair setup. When your lower back isn't properly supported in a seated position, the deep flank muscles compensate for hours at a stretch. Research on sedentary behavior and musculoskeletal pain consistently shows that prolonged sitting time is a significant risk factor for lower back and side pain - even when the total hours of sitting don't seem excessive.

Exercise and recovery patterns. Many people experience flank pain as delayed onset muscle soreness (DOMS) after unfamiliar or intense physical activity, particularly exercises that involve rotation, lateral movement, or heavy lower-back loading (deadlifts, rowing, overhead work). The pain may not appear until 24-48 hours after the activity that caused it - making the connection easy to miss.

Sleep position. Side sleeping without adequate support can place the lower spine in a sustained lateral bend throughout the night. Over time, this can create a familiar, one-sided flank ache that's reliably worse first thing in the morning.

Carrying loads on one side. Regularly carrying a heavy bag, a child, or even a toolbelt on one side creates an asymmetric load that the flank muscles on the opposite side work to counterbalance. This can create chronic tension that's hard to trace back to the habit without deliberate observation.

Could It Be a Digestive Issue?

The colon runs through the flank region on both sides, and several digestive conditions can cause pain that people experience as flank pain rather than abdominal pain.

Conditions worth knowing about:

IBS and intestinal spasms can produce cramping or pain that's felt on the side of the abdomen, particularly on the left (the descending colon and sigmoid sit here)
Gas and constipation can cause a bloated, pressured feeling on either side
Diverticulitis (inflammation of small pouches in the colon wall) tends to produce sharp left-sided pain, often with fever and changes in bowel habits

If your flank pain tends to improve after a bowel movement, or is accompanied by changes in digestion, this connection may be worth exploring.

Are There Other Causes to Know About?

Less commonly, flank pain may relate to:

Shingles (herpes zoster) - which can cause a burning, one-sided pain that precedes the visible rash by several days
Nerve irritation from a herniated disc or pinched nerve in the lumbar spine, which can refer pain to the flank or side
Ovarian conditions in women (cysts, endometriosis) that can cause side pain in the lower flank/hip region

How Do You Figure Out Which Cause Is Yours?

Here's the question most people are really asking, and the one that most articles don't actually answer.

The challenge is that many of these causes produce similar-feeling pain. Muscle strain can feel exactly like early kidney discomfort. Digestive flank pain can be indistinguishable from a muscle ache by location alone. Without paying close attention to the pattern - when it appears, what makes it better or worse, what else is happening when it strikes - it's genuinely difficult to narrow down.

What helps is observation over time. Specifically, tracking:

Pain intensity and location - which side, how severe, sharp or dull, radiating or localized
Timing - morning, evening, after sitting, after exercise, after eating
Activity that day and the day before - desk hours, exercise type and intensity, lifting, sleep position
Hydration - total fluid intake
Digestive patterns - any changes in bowel habits, bloating, or abdominal discomfort around the same time
Stress level - psychological stress increases muscle tension and can amplify pain perception

After two to three weeks of consistent tracking, patterns often emerge that are impossible to spot day-to-day. Does your flank pain reliably follow a heavy exercise session by 24-48 hours? Is it worse after long desk days? Does it ease on weekends when you're more active? Does it correlate with digestive symptoms?

That's the kind of specific, personal information that turns "I have flank pain and I don't know why" into "I know my pattern and I know what to test."

DietSleuth is built for exactly this kind of investigation. While it's best known for finding patterns between food and symptoms, the same tracking and AI pattern-recognition approach works for behavioral and physical symptoms too - logging your daily activities, hydration, posture habits, and pain alongside food and letting the AI surface correlations you'd never catch manually.

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When Should You See a Doctor?

Tracking is useful for understanding patterns in mild to moderate, recurring flank pain - particularly when muscle or behavioral causes seem likely. But some presentations of flank pain need medical evaluation promptly.

See a doctor or seek urgent care if you experience:

Severe or rapidly worsening flank pain
Pain accompanied by fever, chills, or feeling systemically unwell
Blood in the urine
Pain that radiates to the groin or lower abdomen with nausea and vomiting
Difficulty urinating or painful urination
Unexplained weight loss or fatigue alongside persistent flank pain
Pain following a fall, accident, or direct trauma to the back

These presentations can indicate kidney stones, kidney infection, or other conditions that need prompt medical attention. When in doubt, get checked out.

What Can You Do Right Now?

If your flank pain is mild and you suspect a behavioral or postural cause, a few practical starting points:

Check your chair setup. Your lower back should be supported, your feet flat, and your screen at or near eye level. A rolled towel behind the lumbar spine can make a meaningful difference.
Take regular movement breaks. Standing, walking, or gently stretching every 45-60 minutes interrupts the sustained muscular load that desk sitting creates.
Try sleeping with a pillow between your knees if you're a side sleeper - this keeps the lower spine in a more neutral position and takes tension off the flank muscles.
Review your exercise load. If your pain follows activity, consider whether you've increased intensity or volume recently. Building back up gradually gives your muscles time to adapt.
Drink more water. Adequate hydration supports kidney function and may reduce the kind of low-grade kidney discomfort that dehydration can contribute to in some people.
Apply heat to the area. For muscular flank pain, gentle warmth can help relax the quadratus lumborum and surrounding muscles.

None of these are guaranteed fixes - but combined with tracking, they give you variables to test and observe over time.

You may also find these articles useful if you're exploring related symptoms:

The Bottom Line

Flank pain is common, and in most cases the cause is something behavioral or mechanical rather than a serious medical condition. Muscle strain from sitting, sleeping position, exercise load, and lifting patterns are among the most frequent drivers - and they're also the ones most people overlook because they don't show up in the standard medical explanations.

The key to finding your specific trigger is observation over time. Tracking your pain alongside your activities, posture habits, hydration, and digestion - and looking for patterns across several weeks - is the most reliable way to move from guessing to knowing.

This content is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your healthcare provider before making changes to your health routine or if you have concerns about your symptoms.

Sources

Mayo Clinic. Flank pain: Causes. https://www.mayoclinic.org/symptoms/flank-pain/basics/causes/sym-20050902
Cleveland Clinic. Flank Pain. https://my.clevelandclinic.org/health/symptoms/24760-flank-pain
Baradaran Mahdavi S, et al. Association between sedentary behavior and low back pain: A systematic review and meta-analysis. PLOS ONE, 2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC8767074/
Casiano VE, Sarwan G, Dydyk AM, Varacallo M. Back Pain. In: StatPearls. National Library of Medicine, 2024. https://www.ncbi.nlm.nih.gov/books/NBK538173/
Healthline. What's Causing My Flank Pain? https://www.healthline.com/health/flank-pain

Flank Pain: What's Causing It and How to Find Your Specific Trigger