Little sore lump under armpit?

Hidradenitis suppra. Maybe this is not the first time you get it. Sounds like hidradenitis supprativa, which is a form of acne, involving hair follicles, sebaceous glands, and sweat glands. It often enlarges, breaks open and drains pus. Scarring may result. you may need antibiotics, anti-inflammatory medicines and sometimes surgical incision and drainage. Losing weight and wearing loose clothing can help.
Abscess. Under the arm is a common location for skin infection or abscess. You definitely need to see a doc for possible antibiotics. It could also be an inflammed lymph node.