Pain in my underarm?

So I’m 19 years old and female. For a while now I’ll wake up with a pain in my underarm, almost like when you pull a muscle. However, here’s the thing, it comes and goes, one morning it’ll be sore, and the next it won’t, and it won’t be for about a week, it also happens in both armpits, but one at a time if that makes sense. Other than that , I have no other symptoms of breast cancer or anything else. I just thought it was sore because I’ve stretched to hard? Is that possible??

Also this morning morning, I woke up and was fine, but throughout the day it started to get sore, and I’ve just got in the shower and noticed that there was a red mark, about the size of a penny where it hurt. But no lump or anything, feels like my other armpit. 

I also feel like it comes when I’ve not shaved my armpits in a couple of days, and they start to get a bit hairy, and I’ll shave and the next morning I’ll be fine. 

I know none of you can diagnose me, but has anyone else experienced something similar?? Or give me some advice?? 

Thanks