Sore under armpits from period???

My left arm pit has been sore off and on for a week, but today it seems to be mostly tender/sore. I read many things saying it's your lymph nodes & it happens especially when you're gonna get your period. I'm supposed to get mine tomorrow, so I was wondering if it could be this??? I know it's been sore under there before but I never worried about it, but know I am for some reason...is this normal at all!??