Hi psychocalipso,
6 years is a long time. I can understand your pain. You were 19 and 20 when you started dating. I changed a lot in those college years. My boyfriend at the start of college and I really grew apart. College is a big time for change. Your girlfriend has just moved out of her parents' home. That is an enormous moment in life and for me the first time I felt truly free. I had an impulse to try new things at that moment. Stuff I was not free to do in my home, and that included some sexual promiscuity for me. She is flirting with men. She is a victim of sexual abuse from a male cousin. is it possible she is questioning her sexual identity and now has the freedom to experiment without parental judgement?

This is a hard conversation to have, but I think you need to initiate a serious conversation with your girlfriend. You deserve to know exactly what is going on. This flirting does not sound harmless, it ended in your slapping her! If you have never seen this behavior in 6 years, something is going on. Get yourself to a calm place and have a heart to heart