Cheated? or not?

I need some help... I have been in a relationship for the past 3 years. About 2 weeks ago I went on a trip with my family and had to leave without my girlfriend. I had a bad feeling during my trip so I checked the phone bill to see what my girlfriend was up to. I found that someone was sending text messages to my GF (I know who that person is and that person is bad news!). I confront my GF about it, she got mad that I checked the phone bill and decided not to talk to me for the rest of that night. Later On, I find out she went out with that person that was texting her along with that persons girlfriend. I come back from my trip and also see a call from that person on my home phone. I confronted my girlfriend about it and all she said was that she swore to me nothing happened between them, and the only reason why she went out without me was because she felt she needed some space. It has been two weeks since this occured and I am also noticing she is wanting to have sex differently than weve ever done it before. It's killing to think I don't know the truth about what actually happened that weekend that I was out of town. I do not know how to handle this situation. I dont want to keep bringing it up with my girlfriend because the topic makes her feel uncomfortable and she says I bring that up too much and that I should get over it because nothing happened. I cant help but to doubt things.... what should I do?