I have a lump in my left armpit. what can cause it and how do i get rid of it?

Infection anb cancer: Lumps in the area of lymph nodes that drain that area suggest an immunological reaction which is most likely a bacterial infection. The most common cause is the over use of anti-perspiriant in the underarm area. These same lymph nodes will trap cancer cells which grow and enlarge giving you that lump feeling. A helpful finding is the tenderness which is more likely caused by infection.