Post-mastectomy underarm boob?

I had a roll of fat under my arm after right sided mastectomy, because the weight of my removed breast was no longer there to pull it forward. It was uncomfortable because it was just where the top side part of my bra sat, so it couldn't decide if it wanted to stay in or out, especially if I lifted my arm. So I had a scar revision about 2 years after the mastectomy, and feel much more comfortable now, although it did mean the scar was extended a bit further around towards my back to give a smoother transition. I was told this would happen.
It is early days for you, and yours might be swelling, fluid, or fat (fat is not a dirty word, we've all got it!).
Good luck with your recovery.