Deodorant can also do this. Watch the type of deodorant you use some deodorants can cause under arm swelling and soreness. Try natural stuff for awhile like burt bees or perhaps dove. Also razors can do this. make sure you are using a fresh razor.
Hi,
Sounds all to familiar...have you been doing any yard work lately?? Are you, predominately, right handed? After raking and planting my gardens and flowers, I had the same issue. Pain in my right armpit...thought it was some kind of mass...as there was a bulge too. BUT...found out it was the muscle, overworked and inflammed and sore. Take a break...apply cold compresses (if you can stand it) and massage the area.
Best wishes!!!
relax,have fun and get sum rest........