Hey Dirtyman. No bro, I have'nt gone thru that, but I have had friends that had to experience the exact thing that you're going thru right now.
I know it sucks bro, but there's no sense in letting it bum you out. The damage is done, now it's just a waiting game.
Try to keep your spirits up and not let it occupy your mind all the time. Chances are, that you are not the only one that a got a letter from her. She is probably trying to just figure out who the daddy is, if she was sleeping around like you were told she was.
Just try to keep it together and hang in there. Don't make yourself sick over it.