Why was she talking to me and another guy at the same time?

Sorta long but I'd appreciate any possible explanations...

My ex girlfriend, we'll call her D, and I started talking again recently after multiple attempts to still be in each others' lives. The last time we had talked was in October and it ended with a very nasty argument that resulted in us not talking to each other for over one month and a half. Anyway, I apologized, and gradually D started talking to me more and more and even texting me.

Anyway, during this same time she's talking to some other guy who happened to be my friend. The prior argument was a result of this guy, and when we started talking again she exposed him to me and was telling me he was still trying to talk to her but would avoid her in person, etc. Anyway, whenever we talked about him or other guys she never admitted to even liking him. Whatever, during Winter Break we start talking a lot more, and she even started texting me first or whatever and we'd send snapchats back and forth. We'd tease and laugh at each other and she'd send me snapchats like 'You're ugly', 'You're not cute', or what have you and whatever. I had the impression she liked me.

During the end of winter break she stopped contacting me for a period of around a week, which ended after she texted me. The very next day which was when school resumed again she texted me as well, and we ran into each other during study hall and she sat next to me and we talked... She was touching me, etc. And I walked her to class even and we were laughing the whole time.

Anyway that very same day I found out that during Winter Break she had madeout with some guy she 'supposedly' had thought she liked. And I confronted her about it. She basically admitted to kissing him and then proceeded to say that I was stupid for thinking she liked me, that me getting mad at things like that was why she broke up with me in the first place, and that she was just being a good friend by texting me lol.

By the end of the argument though, when I said I was disappointed in her for doing something with a guy she barely knew for a month, she proceeded to say

"I thought you cared about me"

"I hate you"

"You only want to see me feel bad and cry and stress and feel horrible."

"You're the most selfish and arrogant person I've ever met." etc.

The funny thing is that ever since we started talking again she'd get mad at me for petty things I'd do, and even had the audacity to get mad and call me immature when I told her not to talk to me again after I found out what she did. I basically felt played even though we weren't going out.

Anyway basically I just want to know why she did this. Why would she talk to me and flirt with me while she was making out with another guy, then lose interest in him and all of a sudden start texting me and what-not a week later? Why deny things?

+1 y
The guy she madeout with was the same guy who was my friend and she tried to expose*
Why was she talking to me and another guy at the same time?
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