Am i Overreacting??

So my boyfriend and I have been together for a while. We don't really fight and we don't really have problems. We had one over a girl that was actually my best friend and her and him were a little too close. She was blowing up his phone all the time, calling him constantly, interrupting our conversations so that he would talk to her instead of me, making up random reasons to be upset (especially with her boyfriend) so he would come to her rescue and make her feel better, etc. Well we talked about it and decided that he needed to keep his distance from her cause things were getting a little too close for comfort. I noticed today that him and I have a mutual best friend on Snapchat. Usually this wouldn't bother me except the fact that my only best friends are him, Her, and another girl I'm friends with. Immediately my stomach dropped because he's not really friends with the other girl. I haven't said anything to him about it and I don't know if I should. Any advice would be greatly appreciated. Thank you