Have you gone to the dr's yet? I don't want to scare you, and it may not be what you have....but, when my brother was 19 yrs old he developed a lump under his armpit. Went to the dr's. Found another lump above collar bone. Turned out to be non-hodgkins lymphoma.
Please have it checked out. Like I said, It may not be what my brother had, but you don't want to mess around with lumps in that area.
Just wanted to add that my brother is healthy and 37 yrs old now!!
He was treated at the Cleveland Clinic and they did a wonderful job!!