Why does it sometimes feel sore around the arm pit during pms?

Variable: PMS seems to vary by each person and over time. Some never have it at all. The descriptions often are likely to be shared by friends and relatives, but yours can be unique. You are more sensitive at this time and since breat sensations are common, and armpits are nearby, perhaps that's the link. See your Dr if it's too annoying or you're worried.