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:)
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?