@kabash since my lumpectomy (May 2019) I have had a more prominent swollen underarm when I raise my arm. Not a lump so much as a swollen area which stuck out a bit. The underarm goes out instead of in. I had a lymphodema massage before Christmas (preemptive, I don't have signs of lymphoedema) and the therapist suggested it might be a seroma. I swam in the sea a lot the last few weeks using my arms to swim over the waves, quite physical, and I think it has gone down a little and I'm hoping it will resolve by itself. That area was radiated in November and perhaps that has affected it too. Good luck with yours. Thanks for posting as I've been wondering if my thing was common.