Pain under the armpit usually comes from something minor: a pulled muscle, an irritated hair follicle, or swollen lymph nodes fighting off an infection. The armpit is a busy intersection of muscles, nerves, lymph nodes, and skin folds, so several different problems can produce pain in the same spot. Understanding what yours feels like and what else is happening in your body can help you figure out the likely cause.
Swollen Lymph Nodes From Infection
The most common reason for armpit pain is swollen lymph nodes. You have a cluster of 20 to 30 lymph nodes in each armpit, and they act as filters, trapping viruses, bacteria, and other invaders before they spread through the body. When your immune system ramps up to fight an infection, these nodes fill with immune cells and swell, sometimes becoming tender or outright painful to the touch.
The infection doesn’t have to be in your armpit. A cold, strep throat, an ear infection, mononucleosis, or even an infected tooth can trigger swelling in your armpit lymph nodes. Skin or wound infections on your hand, arm, or chest are especially likely to cause it, since lymph fluid from those areas drains directly through the armpit. The pain typically feels like a tender, marble-sized lump that moves slightly when you press on it. It usually goes away on its own once the infection clears, though that can take a couple of weeks.
Muscle Strain
The pectoralis major, the large fan-shaped muscle across your chest, attaches near the armpit and is a frequent source of underarm pain. This muscle pulls your arm forward, rotates it inward, and draws it toward your body. It’s most vulnerable to strain when it contracts while being stretched, such as during the lowering phase of a bench press. Other common triggers include throwing motions, overhead lifting, skiing, and contact sports like football or rugby.
A mild pec strain feels like a dull ache or tightness that worsens when you push, lift, or rotate your arm. A more serious tear produces sudden, sharp pain along with bruising and visible changes to the contour of your chest or armpit. If you recently started a new workout, moved heavy furniture, or did any repetitive overhead activity, a muscle strain is a likely explanation. Rest, ice, and avoiding the aggravating motion are usually enough for mild strains. If the pain is severe or you notice a visible deformity, that warrants imaging to check for a tear.
Skin Conditions
The armpit is warm, moist, and full of friction, which makes it prone to skin problems that hurt.
Hidradenitis suppurativa is a chronic condition that causes painful, pea-sized lumps under the skin in areas where skin rubs together, especially the armpits, groin, and buttocks. It typically starts with a single painful lump that persists for weeks or months. Over time, more bumps may form, and some break open and drain pus. In advanced cases, tunnels form under the skin connecting the lumps. If you get recurring painful bumps in your armpit that take a long time to heal, this condition is worth discussing with a dermatologist.
Contact dermatitis from a new deodorant, soap, or laundry detergent is another common culprit. The skin turns red, itchy, and sometimes painful. Razor burn and ingrown hairs from shaving can also produce tender, inflamed bumps. These causes are usually easy to identify because the pain is shallow, right at the skin surface, and often accompanied by visible irritation.
Nerve-Related Pain
A network of nerves called the brachial plexus runs from your neck through your armpit and down into your arm. Compression or irritation of these nerves can cause pain that feels like an electric shock or burning sensation shooting through the underarm area. You might also notice numbness, tingling, or weakness in the arm or hand.
This type of pain is more common in people who carry heavy bags on one shoulder, sleep with an arm pinned overhead, or have poor posture that compresses the space between the collarbone and first rib. The pain tends to be sharp or zapping rather than achy, and it often radiates down the arm rather than staying localized in the armpit itself. If your armpit pain comes with numbness or weakness in your fingers, nerve involvement is a strong possibility.
Breast-Related Causes
Breast tissue extends into the armpit, so conditions affecting the breast can show up as underarm pain. Hormonal fluctuations during the menstrual cycle commonly cause breast and armpit tenderness in the days before a period. This type of pain is usually dull, affects both sides, and resolves once your period starts.
A new lump in the underarm area is listed by the CDC as one of the warning signs of breast cancer. That said, most breast lumps turn out to be benign. Fibrocystic breast changes and cysts are two of the most common causes. A cancerous lump is more likely to be hard, irregularly shaped, and fixed in place rather than movable. Other signs that raise concern include skin dimpling, nipple changes, or discharge. A lump that doesn’t go away after a few weeks deserves an evaluation regardless of what you think it might be.
Heart-Related Pain
Armpit pain is not a classic heart attack symptom, but it falls within the zone of referred pain from the heart. Angina and heart attacks typically produce pressure or discomfort under the breastbone that can spread into the throat, jaw, and one or both arms. People often break out in a cold sweat and feel short of breath, lightheaded, or nauseated at the same time.
If your armpit pain came on suddenly and is accompanied by chest pressure, shortness of breath, or cold sweats, treat it as a cardiac emergency. Isolated armpit pain without these other symptoms is very unlikely to be heart-related.
How to Narrow Down the Cause
The character of the pain and what surrounds it usually points you in the right direction. A tender, movable lump that showed up while you’re fighting a cold is almost certainly a lymph node. Pain that worsens when you push or lift and started after physical activity suggests a muscle strain. A visible bump with redness or drainage points to a skin condition. Shooting or electric pain with numbness leans toward a nerve issue.
Pay attention to timing and triggers. Pain that lines up with your menstrual cycle, appeared after switching deodorants, or followed a new exercise routine gives you a strong clue. Pain that has persisted for more than two weeks, is getting worse rather than better, or involves a hard lump that doesn’t move should be evaluated by a doctor. The same goes for armpit pain paired with unexplained weight loss, night sweats, or fatigue, as these combinations can signal infections or conditions that need further workup with blood tests or imaging.