Holy ****...for real, do something about this other than coming to the boards...do not let someone take advantage of you to that extent...ever. Even if the guy was 6'3 and in my room boinking id still open my mouth...thats just common sense.
If you come home and see her car there you know she's probably there with him, so before going in there call up some friends as witnesses just in case he tries anything again, if he does...have you're good friend Mr. Louisville slugger with you, it was self defense, with multiple friends there as witnesses. Five or six of your friends stories to one girl's (assuming she lies) won't hold much weight for her...
Then abuse the **** out of the girl (verbally), tell her to lick your nut sweat (who knows, she might take it literally and go for it)...then call the cops and tell the truth, you came home to chill with some friends, found her and this guy in the room, he came after you and you defended yourself..