AIO - GF asked to hang out with a guy she had a fling with in the past

My GF (20) asked to hangout with a male friend. The problem is, she had a fling with him in the past. My understanding is that they were never official and it was just "dumb lust".

To make things worse, she asked me via text if I would have a problem with it and she let me know that her gf would be there too. I told her I would have a problem with it being that I know their history. She cancled on him and told him her "boo thang" doesn't want them to hang out. I guess he didn't respond to her. She assured me that she no longer has feelings for him and that she just saw him as a friend.

We've been dating for 3 months. Im thinking about calling it quits. Not sure if I'm over reacting or not. Is this red flag territory?