[PSY]

I think the most important thing to remember is that he's dating YOU. He didn't settle for less because your best friend was "taken" - he CHOSE to date you because he LIKES you. =) Sure, he may have some feelings for your friend, but that's fairly common. You'll probably develop crushes on other boys from time to time, but ultimately, you'll conclude that they're not "worth it", and you'll want to stay with your boyfriend.

You said your best friend used to like your boyfriend. Did she say that? If so, why not address your concerns with her in private? You could say something like, "Hey, I know you said you don't like my boyfriend anymore, and I believe you... but the constant flirting is making me feel insecure anyway. Could you please tone it down a bit, for my sake?" Chances are, your friend doesn't even realize what she's doing, and she'll be more than happy to ease up on the flirting, if you ask her politely and don't make it about how she's "bad" for flirting with your boyfriend.

As for your boyfriend, it's not good to assume anything. Maybe he's attracted to her, maybe he's not. It could be a fleeting crush, or it could be something that's more persistent. What I would do is bring up the subject VERY casually: "Hey, have you ever had a crush on (best friend)?" See what he says. Chances are, he'll deny having any feelings or her (or he'll say he does/did, but it's not a big deal). So long as he's not thinking about acting on his feelings, you have nothing to worry about! Remember: he CHOSE YOU! =)