Hello,
It may be breast tissue, swollen lymph nodes or an infection. I'd encourage you to see your ob provider to evaluate you.
Best wishes,
Dr. Downing
I got that under my right arm with my last pregnancy 12 years ago. turns out it was breast tissue that migrated there. I had it removed about 4 years ago, but it just came back. I hate it cuz I can't wear tank tops. But mine is sore too. best of luck ~d