My girlfriend slept with my best friend?

Beeefore we were together... So I'm extremely in love with this wonderful girl that lives far away. We constantly skype and were constantly messaging each other using our phones and what not. Well she came out to where I live to visit with her friends. Well one night me and my friend were out at a bar where we started talking to this girl. (my current girlfriend) well he was out looking to get laid whilst I was just trying to have a few drinks. Anyways long story short. My best friend hooked up with a one night stand with my current girlfriend before we were dating. The question is. What should I do? I mean I love this girl but I constantly see them two together for a one nighter and it eats at me. I try to ignore it or says oh well it didn't mean anything it was a one night stand but it still bugs me. I just don't know what to do.