I have a painful lump in my lower armpit. what could it be?

Infected node ?: We all have lymph nodes in the armpits. These can get infected. Nodes that swell can mean various things so seeing a physican is useful..They will check for other lymph nodes and likely figure it out.

Infection: You have the odor producing glands in that area and these can become infected by skin germs. Localized moist heat may help the body self heal, but occasionally these need to be drained or treated with an antibiotic.You also have lymph nodes in that location that handle issues for arm & shoulder.These can also become swollen & self heal but occasionally need treatment.