<human>: Can you summarize the given text in a few sentences?

So, I had a girlfriend for 6 months at university, but we live in different cities over the summer. We agreed it'd be best if we broke up over the summer, since it's 4 months apart, but decided to stay friends. We broke up at the start of May.

After we break up, we transition into being friends nicely. About a month after the breakup (this morning), she tells me a guy at her job, has been flirting with her. I give her some advice about how to deal with it. Just as the conversation is coming to an end, she tells me, "oh, also, I had sex with him last week."

So of course I'm taken aback, since that's not really something that's ever okay, and I didn't even ask to hear about it.

The worst part is, that very weekend she was upset at me for "seeming as if I moved on", which made me feel like total shit. When realistically, she was already fucking some other guy.

We were always very open with each other, but at this point I'm starting to feel like she's just trying to make sure I don't see other people, while she goes around hooking up with strangers.

My question is, what do I do in this situation? I thought we were good friends, but now it just makes me question everything. I'm more upset with that she lied to make me feel bad when she was banging another guy, and having never been in this position before, I don't know what to do.

Finally, I'm a group leader with her for something in September when school starts up again, and I don't know how I'm going to be able to face her. Please help!
<bot>: broke up with girlfriend, she got pissed at me for seeming like I moved on, turns out she was fucking some guy all along. Help with how to deal with this.