Left Underarm Pain: Differentiating Between Breast Changes and Heart Strain

Experiencing pain under left armpit can be worrying. Because the left side of the body is often associated with heart problems, it's natural to feel concerned. At the same time, the underarm (axilla) is closely connected to breast tissue, lymph nodes, muscles, and nerves—many of which can cause discomfort that is not heart-related.

Understanding the possible causes can help you respond appropriately without ignoring something serious. Below, we'll break down the most common reasons for pain under the left armpit, how to tell the difference between breast-related pain and possible heart strain, and when to seek medical care.

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Understanding the Anatomy of the Left Underarm

The underarm area contains:

• Lymph nodes
• Breast tissue (including the "tail" of breast tissue that extends into the armpit)
• Muscles (such as the pectoral and latissimus muscles)
• Nerves
• Blood vessels
• Sweat glands and hair follicles

Because so many structures are located here, pain under left armpit can have several possible explanations—most of which are not life-threatening.

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Breast-Related Causes of Pain Under Left Armpit

The breast extends into the underarm area through what's called the "axillary tail." This means breast-related conditions can cause discomfort that feels like underarm pain.

1. Hormonal Breast Changes

Hormonal fluctuations during:

• Menstrual cycles
• Pregnancy
• Perimenopause
• Hormone therapy

can cause breast tenderness that radiates into the armpit.

What it feels like:

• Dull, aching soreness
• Swelling or fullness
• Tenderness that worsens before your period
• Often affects both sides, but can be one-sided

This type of pain is usually cyclical and improves after menstruation.

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2. Breast Infections (Mastitis)

More common during breastfeeding, mastitis can also occur in non-breastfeeding women.

Symptoms may include:

• Pain under left armpit or breast
• Redness or warmth
• Swelling
• Fever
• Flu-like symptoms

Mastitis requires medical treatment, often antibiotics.

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3. Enlarged Lymph Nodes

Lymph nodes in the underarm can become swollen due to:

• Infections (even a minor cold or skin infection)
• Vaccinations
• Immune responses

Signs of swollen lymph nodes:

• Tender lump under the skin
• Pain that worsens when pressing on it
• Temporary swelling that improves over days to weeks

If lymph node swelling persists beyond a few weeks or continues to enlarge, a doctor should evaluate it.

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4. Breast Cancer (Less Common, But Important)

While most underarm pain is not cancer, persistent symptoms should never be ignored.

Breast cancer may spread to nearby lymph nodes, including those in the underarm.

Possible warning signs:

• A firm, painless lump in the breast or underarm
• Skin dimpling
• Nipple changes
• Persistent swelling
• Unexplained, ongoing pain on one side

It's important to note that pain alone is rarely the only symptom of breast cancer, but new or unusual changes should be checked by a doctor.

If you're experiencing unexplained discomfort and want to explore whether your symptoms might be related to breast tissue, try this free Breast pain symptom checker to help identify possible causes before your doctor's visit.

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Heart-Related Causes of Pain Under Left Armpit

Because the heart sits slightly left of center in the chest, pain from heart strain or reduced blood flow can sometimes radiate to nearby areas—including the left arm and underarm.

1. Angina (Reduced Blood Flow to the Heart)

Angina occurs when the heart muscle doesn't get enough oxygen-rich blood.

Common features:

• Pressure or tightness in the chest
• Pain that spreads to the left shoulder, arm, neck, jaw, or underarm
• Symptoms triggered by exertion or stress
• Relief with rest

Angina is a warning sign of heart disease and requires medical evaluation.

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2. Heart Attack (Medical Emergency)

A heart attack occurs when blood flow to part of the heart is blocked.

Possible symptoms include:

• Chest pressure, squeezing, or fullness
• Pain under left armpit or down the left arm
• Shortness of breath
• Nausea or vomiting
• Sweating
• Lightheadedness

Women may experience subtler symptoms than men, including:

• Fatigue
• Back pain
• Jaw pain
• Mild chest discomfort instead of severe pain

If pain under left armpit occurs along with chest discomfort, shortness of breath, or nausea—seek emergency medical care immediately.

Do not wait to see if symptoms improve.

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Musculoskeletal Causes (Very Common)

In many cases, pain under left armpit is caused by muscle strain or irritation.

Common triggers include:

• Heavy lifting
• Upper body workouts
• Poor posture
• Sleeping in an awkward position
• Repetitive arm movements

Muscle-related pain often:

• Worsens with movement
• Feels sharp or pulling
• Improves with rest
• Is tender when pressing on the area

Unlike heart-related pain, musculoskeletal pain is usually reproducible—meaning you can trigger it by moving or pressing the area.

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Nerve-Related Pain

Irritated or compressed nerves in the neck or upper spine can cause pain that radiates into the underarm.

This type of pain may feel like:

• Burning
• Tingling
• Shooting pain
• Numbness

Cervical spine issues or pinched nerves can mimic more serious conditions but are generally non-cardiac.

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Skin and Sweat Gland Conditions

Sometimes the cause is simple and localized:

• Ingrown hairs
• Shaving irritation
• Cysts
• Boils
• Hidradenitis suppurativa

These conditions usually produce visible skin changes, tenderness, or localized swelling.

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Key Differences: Breast vs. Heart vs. Muscle

Here's a simplified comparison:

More Likely Breast-Related

• Cyclical pain tied to menstruation
• Swelling or tenderness in breast tissue
• Localized underarm lump
• Pain that changes with hormonal shifts

More Likely Heart-Related

• Chest pressure or tightness
• Pain spreading to left arm, jaw, or back
• Shortness of breath
• Nausea or sweating
• Triggered by physical exertion

More Likely Muscle-Related

• Pain worsens with movement
• Recent strain or exercise
• Tender to touch
• Improves with rest

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When to Seek Immediate Care

Call emergency services if pain under left armpit occurs with:

• Chest pressure or tightness
• Shortness of breath
• Fainting
• Sweating or nausea
• Sudden, severe pain

These could signal a heart attack.

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When to Schedule a Doctor's Appointment

See a healthcare provider if you notice:

• Persistent underarm pain lasting more than a few weeks
• A new lump in breast or armpit
• Swollen lymph nodes that don't shrink
• Unexplained breast changes
• Pain that keeps returning

Early evaluation provides peace of mind and allows prompt treatment if needed.

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Final Thoughts

Pain under left armpit can stem from many causes—most of them not life-threatening. Hormonal breast changes, muscle strain, and temporary lymph node swelling are far more common than heart problems.

However, because the left side of the body is associated with cardiac symptoms, it's important not to dismiss warning signs such as chest pressure, shortness of breath, or radiating pain.

If you're concerned that your underarm discomfort might be connected to breast tissue changes, you can use a helpful Breast pain symptom checker to get personalized insights into what might be causing your symptoms.

Most importantly, speak to a doctor about any persistent, unusual, or severe symptoms—especially anything that could be serious or life-threatening. Prompt medical care saves lives, and getting checked is always better than wondering.