A small bump in the armpit. There are several common causes for this. First, sebaceous cysts. A so-called sebaceous cyst is a sebaceous gland that can drain outside the skin through small ducts when it normally secretes sweat. However, the ducts are blocked by sweat or surrounding tissues, so that the sweat cannot be discharged. This gradually collects and a small bump appears, called a sebaceous cyst. Secondly, lymph nodes, which appear enlarged due to surrounding infection, can also be felt as a small bump.