I could really use some relationship advice?

Ok, so I was with my girlfriend for 10 months and we broke up for about a month, but we got back together today. She admired to me to having with another guy while we were apart. I don't know if I should be angry and call it off or not. She did lie about it when I first asked her but then told me, and told me she lied because she was ashamed and didn't want to hurt me. Should I trust her or what? I'm so confused I don't know what to do.