<human>: Can you condense the text into a few key points?

So.... My girlfriend has school friends of course, she was recently introduced by one of them to a guy... According to her school friend, this guy thinks she is cute. Cool, not a problem i'll take it as a whole pride thing knowing someone is interested in my girl. Now they recently made a group chat consisting of my Girlfriend's school friends and this guy as as well as one of her friend's boyfriend. They don't talk to often, but apparently in that group chat they say numerous times about Her and that guy hooking up. Now apparently the guy with the knowledge of her having a boyfriend, decided to message her just a chat between her and him. From what i was told from my girlfriend this guy randomly just said not directly to her, but to the friend that introduced her. "Yeah, im going to be dating her in a couple months", This is 100% a red flag to me.. but my girlfriend does not seem to find it this way. Now for a background on me, before the relationship I was fairly flirty when it came to girls... She knows that, but as soon as we started dating.. She would get upset if she saw me doing it, so i cut that out. Now when they were talking on their own convo, she was apparently flirty.. and she feels there is no harm to it, right now i feel my trust in her is slowly going away. She told me last night that they were talking, so i tried to get it out of her asking about him. I would make a point then she would be upset that i somewhat called her out on something. I had a crappy start to the morning with this situation, and i don't know how to come of it.
<bot>: Girlfriend was introduced to a guy that thought she was cute, she flirted with him... Thinks its harmless but she got mad at me when i would do that.