i felt like i had a large lump of skin under my arm pit after as well. and, i did get a bit of a seroma (fluid) there, which made it feel bigger. and, it was numb, which oddly made it feel even bigger! so, it was like a trifecta of uckiness.
i also had axilliary web syndrome - also called cording. there are a few studies on this syndrome, and for some women, it seems to affect range of motion pretty dramatically. it feels like guitar strings running from the bottom of your arm pit to about half way up your upper arm (if it's extended up). anyway, i ended up a subject in a graduate student's dissertation work at the U of MN... and i learned a bit about it. turns out, they don't really understand it... but it does go away within a few months.
the good news is that now the lumpy part is flat, and the numbness is abating. i know it will never actually go completely away because of the sentinel node biopsy, but it is getting smaller and i am getting used to it. and the seroma is completely gone.
i have two puffy bits on either side of my sternum, and i will likely have a revision to flatten those out. the surgeon who did my mastectomies said it is recommended to wait about six months before doing a revision. everything changes so much in those first few months...
overall, everything gets flatter and smoother with each passing month. i had my surgery in feb, and i feel like everything is doing super well.
so... keep moving - that will help with range of motion and pain and fluid... if the evening set is harder, what if you break it into two smaller sets? and long duration low load is the way to go on stretching... that means hold a very light stretch for a long time. if you feel better in bed, can you do a few of those exercises in bed perhaps?
and keep talking to your health care team for sure! also, many women go to physical therapy after a mastectomy - the physical therapists have a great suite of exercises that help improve things....
good luck. virtual hug.