Dealing with discomfort with other girls

Okay so my bf and I are in a very happy and commited relationship. I have no fears he'll cheat on me or vice versa. That being said, I feel uncomfortable when a certain girl hugs him. They didn't date but she was the only girl he had feelings for in high school and they went to prom together before we met. Not to mention she's gorgeous. But whenever they see each other she hugs him. 
I know the hug is innocent and I have no issues with the girl (except for their history) and I don't get mad. It just makes me uncomfortable.  I just want to stop feeling that discomfort so it can't ruin my mood. Any tips?