Deep or superficial?
If its up in the skin and moveable it is most likely just a skin cyst.
If it is deeper in the armpit between the muscles it may be an inflamed lymph node or fatty tumor. These can feel like a grape deep under the skin. If you are unsure, have an md look at it. If it persists longer than a couple months or begins to grow have it seen right away -- at your age it is most likely benign.