Bump under arm..?

recently iv been getting a small bump under my right arm. at first it was pea sized and showed through the skin.. it would always go away within a week so I wasnt' concerned. they keep comming back now.. one after a nother and now they are getting larger and harder to pinpoint. its alwasy in my right arm and very painfull. I just wanted to know what it may b.. that it keeps comming back. just started within the last fue months and grow bigger everytime..