Ex boyfriend dating a close friend?

My ex is dating a close friend of mine that I’ve known since elementary school. We’re seniors now. I unintentionally started dating a friend of his in dec. 2016, two months after he and I called quits on our relationship of almost 2 years. His friend and I met by having classes together and having many similar interests. I know I shouldn’t be worried about what my ex is up to but I can’t help it. I think them getting together is way to coincidental. Thoughts anyone?