The girl is not meant for you.

So, She had a mattress mambo with a dude in your absence. Even it was for a day, she was emotionally attached with that dude. Similar thing happened to a friend of mine where the guy only wanted a one night stand and stopped receiving calls from the next day and the girl had no options other than patching up with her boyfriend. It would have been equivalent to you (as you did bang hookers) if she had sex with a giagolo only.

Do youself a favor and don't fall for her. It would be easier to forget if you were an American but it WILL HAUNT you for the rest of your life whenever you want to make love to her. Again, If you really want to get over it, you GOTTA beat crap out of that dude.