I had quite a bit of pain and discomfort like that during chemo. For me, too, it felt similar to the pain that led me to discover my tumor, which is very disconcerting.
I did a lot of googling trying to find out if it was bad news, and all I really found was that it was quite common! Many women attributed it to the tumor shrinking, but like you, I felt it in my armpit and not just in my breast where my tumor was.
All of my worry was for nothing, because I went on to have a complete response to chemo (PCR), and I hope you will do the same!
Still to this day, 8 months out of chemo, I get weird tingles and zaps in my breast, even the lymph nodes in my neck. Chemo messes with you in odd ways. My oncologist says it's normal.