the first day i saw it, i thought it could be from being too damp down there, so i used a lot of baby powder to keep it from being too sweaty there...maybe it's too dry now and that caused this?
It does not sound like an STD, it sounds more like youv had sex recently and got a little tear, maybe there wasnt eoungh lubrication, maybe your BF is on the big side (as is mine-so I get these from time to time too). Leave it alone it will heal by itself your only going to irritate it with powder etc!
Take care:)