Post-mastectomy underarm boob?

It’s most likely swelling. Also a little fat under the arm can look different after a mastectomy and axillary clearance, doesn’t quite fit the same way. The other possibility is a seroma, fluid accumulation (your body trying to assist, just like fluid forms on a blister if you scrape your heel). The latter is usually easy to determine and will usually disperse of its own accord. Your surgeon knows about it; if it continues for too long, just give her another nudge! Best wishes.