Overreacting?

My boyfriend will be 20 in January, he's friends with this 15 year old girl.(they used to have feelings for eachother) about two years ago. In my opinion, when you're 14/15 it's a lot different than 18/19. She's told him to leave me, she's had feelings for my boyfriend, and 3 of his friends. The thing between those two is in the past, but it makes me feel uncomfortable when they hangout.. Is it overreacting? I know he wouldn't cheat on me, it's just the whole issue itself