It could be a heat rash or something along those lines. It can happen because your body is naturally warmer there and heat and sweat can build up causing a rash - especially if your clothing is binding. Be sure not to wear a bra to bed & a loose shirt so your skin can breathe. Try using some powder too and see if it helps it clear up. My only other suggestion is to see a doctor.

Hmmm..Well I usually wear a bra to bed so that might be it lol but yeah ill try going without it when I go to sleep for a while and then see what happens, thanks!