<human>: My girlfriend and I have been in a relationship for about 6 years, known each other for 2. We live together and we have been for the past 4 years. She's awesome, and kind, and very loving. I love her and want to marry her, until two weeks ago. She was helping her friend out for a project to model some clothes. I didn't know this until she told me what happened, but I guess the photoshoot was themed as a "dating couple". So, she did some stuff, I assume was something similar to what couples do. When she told me this, I almost lost it. How can she do that to me? At first, it seemed like it didn't bother her, until she asked me if I was upset. Of course I'm going to be upset, she just did what she does with me, with another guy. She started getting upset and crying. She said she didn't know that it would hurt me. How can she NOT know? I feel like I have been cheated on. She also said that she was very sorry, (and I can tell she was very sincere about it) and that it will never happen again. But I don't know if I can look at her the same. 

We also have been cheated on by other people before, her more than me. I have had girls lie to me constantly, and made me question and eventually lose trust in them. So, I'm not sure if I'm just deriving from the past, or she really did cheat on me. I'm confused.

Can you summarize the given text in a few sentences?
<bot>: Girlfriend acted like a couple in a photoshoot, did she cheat on me or am I over-reacting?