My g/f and I broke up becuase....?

she decided to talk to/date other guys before we totally ended it. while we were broken up she had "relations" with other guys at college during a 2 month period. Now we got back together, but neglected to tell me she slept with other guys during the time apart. She thinks it's no big deal. I was enraged. Not only from a health standpoint, but also since it has greatly affected me, my thinking towards her, and our relationship. She says I should just forget about it so it won't make things worse. However, I cant stop thinking about it. Am I wrong, is She wrong, is it my problem or hers, and is our relationship doomed? I need help :(