Jealous of boyfriend’s best friend’s new girl?

I don’t know why I’m feeling like this at all but I just don’t feel the happiest about his friend getting with a new girl? I don’t know if I’m just protective and like it just being us 3 or if there’s something about this girl I’m not sure of yet...or maybe it’s because I know he’s lied to her about how many girls he’s slept with. Is it normal to feel like this at the beginning? I like the girl and obvs he’s a great friend but I’m just unsure about it at the moment.