Breaking Point.

So my boyfriend and I dated for a year and a half and then broke up for a couple months and dated other people. We got back together and have now been together for a long time. Tonight for the first time ever, I saw one of his ex's that he dated while we were broken up. We had just finished getting gas and were preparing to drive away when she pulled into the gas station. She doesn't live in my city which shows why I wouldn't really see her in person. Anyway, he made a comment either calling her a slut or a bitch, but said it in a sentence. I don't remember which one it was or exactly what he said. He then threw garbage out the window and sped away. I feel like he handled this the completely wrong way and still has some kind of feelings towards her. Especially because when we were broken up, he treated me like shit and still loved me and here we are back together. Therefore I feel like that's a huge red flag because if he was over her I feel like he wouldn't have even cared that she was there and that he would've acted like he never noticed her. I'm really torn at how childish he acted.