My girlfriend cheated on me in May?

and I kind of knew it so I had her admit it, because I know when she lies. After that she apologized and said it was a mistake. I know another person was talking to her the other day being a total creep saying they wanted to have sex with her. She still texted the person after the fact and if I didn’t stay with her tonight she was “maybe” going out with them. Do you think it would be out of line to ask her if she’s out with them over text right now? I don't think it would be given the past...but I wanted other peoples opinion's.