According to the Mayo Clinic, pea-sized lumps in the armpit are a symptom of hidradenitis suppurativa, a condition in which the hair follicles become blocked. The cause of hidradenitis suppurativa is unknown, but the condition is linked to hormonal fluctuations and excess weight. Symptoms include pea-sized lumps under the arm, blackheads, leaking sores or tender red bumps in the armpit, between the thighs and underneath the buttocks.
WebMD explains that certain infections can cause the lymph nodes under the arm to swell. Swollen lymph nodes under the arm are referred to as axillary lymphadenopathy and are typically detected during physical examinations or X-rays. WebMD also states that painful lumps in the armpit can be caused by infected hair follicles. In certain cases, infected hair follicles can cause abscesses.
According to Medline Plus, small lumps in the armpit can be caused by a variety of factors that include cysts and skin infections caused by shaving or anti-perspirants, breast infections, cat scratch disease, shingles, herpes zoster, infectious mononucleosis, normal breast tissue, chickenpox and harmless fatty growths. It is important for individuals experiencing unexplained lumps in the armpit to consult with a medical professional, as certain lumps or growths may signify breast cancer or lymphoma.