Hi KH - I would push for a mammogram and an ultrasound. It's possible that it could just be a swollen lymph nodes, but at the same time breast tissue does extend as far as your armpit. It's ALWAYS best to get any lump checked out that doesn't come and go with your period. The ultrasound will detect if the mass is solid or fliud filled (cyst).
Best of luck to you. Please keep us informed.
Angi