Am I over reacting or is she cheating on me?

I completely understand that you are upset about her wanting to break plans with you last minute for some guy that came into town, however, I do think that you are possibly worrying about a problem that isn't even there. If you have no reason to not trust her, then you should trust her. She's not the girl who cheated on you, don't hold that against her. Also, there is nothing wrong with a girl having male friends, or a guy having female friends, as long as it is friends and you can trust the other person, it's fine. Someone who is going to cheat is going to cheat, there is no preventing it. Just treat her the way a woman should be treated and tell/show her how much you love her everyday.