Like you, Toulula, my worst hurts were also under and down my arm, rather than my breast site, which always felt fine and looks very neat - I'm 5 weeks post mx.I had hardly any pain relief in hospital as I didn't need it, but had to start on painkillers for the under-arm pain. Phone the bc nurse if you're particularly worried, though.
I also have problems with raising my arm, despite doing the exercises on my sheet several times daily, and I was told by the bc nurse who was draining fluid last week that it was cording, and she put me down for cancer physio, which I will start next week. I obviously had this problem 5 years ago when I had lumpectomies on my other side, but nobody mentioned cording, and my GP sent me for 2 rounds of physio, which were no help. I eventually found out about cancer physio when chatting to a bc nurse ages later, and when I went there it was totally different to any other physio I'd ever had. She gently stroked from the lumpectomy site right up to my shoulder and over several weeks it improved a huge amount. I wished I'd known about that before I had radiotherapy, as it was agony getting that arm in the correct position. We live and learn. Hugs, Jo x