Answer:
This is a very difficult question to answer. There are many things that could cause a painful bump or lump underneath the skin in your armpit. It could be an infection, a swollen lymph node, a bacterial infection if you shave your armpit, a fatty growth, cancer or any number of things. Here is a useful website at the National Library of Medicine website Medlineplus on armpit lumps
http://www.nlm.nih.gov/medlineplus/ency/article/003099.htm
The best answer to this is to please try to see a doctor or nurse or get to a health center. Treatment will depend on which of the many relatively harmless or serious conditions that could be causing this bump or lump.
Medical Librarian