I don’t think there was anything to get mad about. In high school, it’s hard not to socialize with the same group of people for four years, and since Kasey dated your boyfriend before you did, and is now friends with both of you, it doesn’t sound like it’s out of the ordinary for her to invite you both to a bonfire or a movie in her home. I think you didn’t go, and your natural jealousy got the best of you.
It sounds like it was a group of people who were going to the bonfire, and then went to the movie instead. There’s not a lot of harm in your boyfriend being part of a group that included his ex-girlfriend who is now friends with both of you.