So, I don't know if your girlfriend really cheated on you. Obviously I don't know, I'm just some internet stranger. But I also don't think it's clear from the details you gave us.

What's clear to me though is that there have to be some massive underlying trust issues from your side.

When I found out that it was some people from work that I had never heard of, I pressed her for some details.

What kind of details? She told you she's with some people from work. I don't think that should raise any suspicions...? If your immediate reaction to her saying she's out with some co-workers was to ask her if she's attracted to one of them, then I think that speaks volumes.

She admitted that she was attracted to one of the guys.

Not a problem in my opinion.

Since she was drunk at the time, I asked her if she would have kissed me/ had sex with him if he had made a move on her. She said she would have

Would have if she wasn't in a relationship with you or would have period? That's kind of an important detail. The former would not be a problem for me, the latter most definitely yes.

However, we had plans to visit my sister on saturday (today) and I told her that I would pick her up at 10am from her parents. She said that she would just drive herself. This made no sense and started to arouse my suspicions.

Why did that not make any sense?

I asked her to snap me a picture of her parents house.

If your immediate reaction is to demand proof about where she is instead of asking her what's going on, that speaks volumes too.

I told her she needed to call me or the relationship was over. She didn't call.

I wouldn't have either. That's emotional blackmail.