OP, I had an eerily similar experience a few years ago.
I was in an LTR for 1 year, already bored as hell. Really didn't want her anymore and she knew it deep down.
So one night she goes out with her friends to another city. She invites me, but I wanted to take the opportunity to go out on my own. I told her to have fun.
I see her the next day, and she's acting weird. I tell her I want to know what's up, she starts crying and says she has to confess something. Tells me she kissed another guy in a club, and gave him her phone number (though the guy lived in another state). She just started bawling at this point, telling me how sorry she was.
So I sat there, wondering, "why would she tell me this? She knows I'm already thinking about dumping her, why is she risking the relationship?"
I believe it was her way of manipulating me into chasing after her, of getting me to "want" her again.
I did wonder about whether she actually fuked the guy. Looking back, I don't think she did. But she later admitted to "making out" with him, and I wouldn't be surprised if she let him finger her or maybe even gave him some hand action.
So anyway, I dumped her. When we got back to her place, I went inside and took the lingerie I had just bought for her birthday. I don't remember what I said at the time, but it was something about her being a hor and I'm not paying for her to dress like a slvt for other men.
She was going crazy, it was hilarious. She physically blocked me from walking out the door. I eventually had to move her out of the way. And she kept grabbing me on the way out.
She chased me very hard for a while after that, and I finally took her back after I think 3-4 months. I don't think I've ever had a stronger frame in a relationship than I did with her after that point. She was in complete submission. Of course, I got bored with her all over again in a few months and dumped her again.
Just some thoughts.