Shit, that sucks.
On the plus side, she's probably not someone you'd want to be involved with. Find her boyfriend and let him know she's a cheat (unless she only met him after you guys did the dirty).
Frankly, it feels like I've been cheated on, because it wasn't even a drunk accident or what have you, it was us having a day together over at my house and it got a bit romantic, call me old fashioned, but I didn't think I needed to say 'So wanna be in a relationship with me?' after kissing started xD
Must admit, kind of tempted to send her boyfriend a message, however, I know that won't achieve anything, I mean, yeah, a message saying 'Your girlfriend was in my bed two days before you got together' could do some damage, but that won't make things as they were before, at the end of the day, she's the one that did it, so getting into a fight with her boyfriend would just be pointless, as it's misguided anger. I have a week of shows ahead, anyway, so this couldn't have come at a better time, I can use this week to get over her and keep myself busy, I reckon.