There are many reasons for the growth of small bumps in the armpit, such as skin tags, sebaceous cysts, boils, lipomas, filiform warts, lymphadenitis, and so on. 1. skin lumps: they are soft fibrous tumors, a kind of benign tumors, occurring in the armpits, neck, groin and other folded areas, which can lead to a small pimple in the armpit. 2. Sebaceous cysts: localized hair follicles and sebaceous glands increase the secretion of sebaceous glands, the sebaceous gland ducts are blocked, and the secretion of the glands will cause the local cystic structure to form sebaceous cysts, which can be manifested in the form of armpit pimples. 3. Boils: purulent infection of hair follicles and deep soft tissues, early symptoms will cause subcutaneous lumps, pain, redness, swelling and other phenomena, which will lead to the symptoms of underarm pimples. 4. Lipoma: abnormal metabolic hyperplasia of mature adipose tissue under the skin, benign tumors formed by differentiation, can lead to a small bump under the armpit. 5. Filiform warts: are benign skin abnormalities caused by human papilloma virus infection, which may lead to a small bump in the armpit. 3. Lymphadenitis: the lymph nodes in the armpit are infected by external pathogenic bacteria, which can cause the lymph nodes to be swollen and painful, leading to the symptom of a small bump in the armpit. There may be other reasons why you may have a bump in your armpit, so you need to go to the hospital and get a clear picture of the cause of the problem and follow the doctor’s instructions.