To be honest, the only people who know the truth is her and the other guy from work. The only way that you will ever know the full truth is if she is willing to tell it to you. I would tell her how much it really bothers you. If you think you can handle the full truth and move forward in the relationship knowing the truth, then tell her that. But also explain to her that it’s going to be very difficult for you to continue in the relationship if she isn’t being completely honest. Plus, you wouldn’t want to keep building the relationship on a possible lie. Kind of a sticky situation. I’m sorry you’re in it and wish there was more advice I could give.