Should I suspect him?

Okay, so things have been going fine between my boyfriend, Charlie, and I until recently. He was talking to one of our clmates, and they ended up in an agrument about this girl he used to like, Vicki. He was all defending her because our clmate, Melanie, was saying that Vicki is a bad editor. Okay, so Vicki is the editor of our school's newspaper, and Melanie is co editor or something. Melanie is mad that Vic changed her layout without telling her. Then Charlie said that Melanie is stupid for talking sh** behind ppl's back, esp. with Vic, since Vic worked SO HARD ON IT. I was like sitting right there! Why am I suspicious? Well, Charlie used to like me and Vic at the same time, and I never really trust him after I found out that he flirts majorly with her. I forgave him but, he defended her sooooooooooo pionately right in front of my face!!! What does that mean?! he knows that i get jealous... so why is doing that? Just caught up in the moment cause he never really liked Mel?