According to Healthline, an armpit lump is likely an enlargement of one or more lymph nodes under the arm. The most common, easily treated causes include bacterial infections, allergic reactions to deodorants, viral infections or adverse reactions to vaccines. However, underarm lumps may also be a sign of cancer.
Healthline explains that most lumps are harmless and usually do not require any invasive treatment. Depending on the cause, treatment could include oral antibiotics, a change in lifestyle or simply rest and warm compresses. Anyone with any unknown lump should visit a doctor to determine the exact cause, as it could be an indicator of a more serious underlying condition.