Lumps in the armpits may be caused by swollen lymph nodes in the armpits due to lactation mastitis, fibroadenoma of the paratesticular breasts, folliculitis and other diseases. 1. Swollen axillary lymph nodes due to breastfeeding mastitis: Breastfeeding mastitis is an inflammatory reaction of the mammary glands triggered by milk stagnation caused by various reasons, with or without bacterial infection. Inflammation can spread through the lymph and lead to swollen lymph nodes in the armpits, producing underarm bumps. 2. Fibroadenoma of paratesticular gland: paratesticular gland can appear in the axilla through the nipple to the inguinal part of the primitive mammary gland on any part, clinically the most common is in the axilla, paratesticular fibroadenoma clinically manifested as localized lumps, the lumps have clear boundaries, smooth surface, medium texture, mobility is still possible, may or may not be accompanied by pressure pain. Therefore, it can produce underarm lumps. 3. Folliculitis: underarm hair follicles are easily inflamed, folliculitis is often caused by Staphylococcus aureus, occasionally other pathogenic bacteria. Signs of folliculitis are superficial pustules or inflammatory nodules around the hair follicles. Lumps may be palpated in the axilla. Bumps under the armpit may also be other causes, it is recommended to go to the regular hospital, clear causes under the guidance of the doctor for standardized treatment.