Is it normal for me to feel sick about this?

Ok. My girlfriend and I had been together for a little over 2 years when we called it quits. Our relationship was great in a lot of ways, our sex life has always been A+ and the only reason we broke up was because we both have bad tempers and fought way too often.

We were separated for nearly 4 months and agreed not to talk for 2 of them. We made it to about 1.5 months of not talking, at which point we started hanging out again. She had always been bi-curious, wondering if she was interested in men as well as women. I was her first partner in every sense of the word.

We ended up getting back together a few days ago after a long, heartfelt conversation involving many tears and feelings. I then found out through a series of unfortunate events that she had fooled around with a guy while we were separated (she went down on him, he didn't touch her). Long story short, he used her to get off and then basically told her to take a hike after they had been talking for a couple weeks. I feel outwardly angry that he used her because despite my obvious jealousy I love her more than anything and could not bear to see her so hurt. She feels used and regrets the choice in the person, but not the action itself as it helped satisfy a long-term curiosity of her. She assures me it meant nothing and that I have nothing to worry about. We had also previously lived together, and I found out that it had happened in the room that used to be mine (we lived with her parents so we had separate rooms).

While I hold no resentment towards her for her curiosity whatsoever, I can't shake this awful feeling in my gut when I think about it. Every time I'm in the dark, my mind flashes the image of her pleasuring him and I want to throw up. Every time I'm alone I get heart palpitations and I feel claustrophobic. I just need to know if it's normal for me to feel panicky/anxiety whenever I think about it? I feel weird in that room, and I feel weird when we're being sexual in the sense that when she tries to touch me I get very uncomfortable and have to stop. I don't want her to feel she is being punished, as I have had about 7 partners in the past and she managed to get over MY history and I know if I had been in her situation I would have done the same thing. I just really need to know what I can do to stop feeling like this because I love her very much and we both want it to work out this time around.