I'm went through something similar.

I recently met a girl at a bar. We got along fabulously (which is rare, because I don't normally connect with people that quickly). We had multiple common interests, and there were sparks flying between us. We dance and drank the night away. I got a huge hug, and a phone number at the end of the night.

We chatted via text and on the phone. A couple days later we met up for a dinner date, and made out afterwards. After leaving the restaurant, we ran into a mutual friend. The mutual friend asked her about "Dave". I had no idea who "Dave" was. She quickly changed the subject, and excused herself about 10 minutes later.

I tracked down the mutual friend, and started asking questions. I found out she has a boyfriend that she's been dating for a couple months. She never said anything about it to me.

I confronted her about this. She said she'd break up with him for me. I told her not to bother, because I couldn't trust her.

If she could cheat on him, why wouldn't she cheat on me?