A little over a month ago, a lump started growing in my left armpit. It was hot, tender, painful, and eventually about the width of a quarter. About a week later, it started decreasing in size, until it shrunk to about the size of a dime. By then it was no longer hot or painful, barely noticeable. Then at about 3 weeks it began growing again, not painful, but noticeably larger. After a few days, it started shrinking again. As of right now, it is currently increasing in size, but not extremely. It is not painful or bothersome, but I'm worried about WHAT it is, WHY it's not going away, and WHY it keeps changing in size. It's not an ingrown hair--there's no head. Could it be a tumor? I didn't think tumors grew and shrank, though. For reference, I am a 17 year old girl, with no medical history of abnormal diseases or conditions.