Very good, dear! You're taking a step forward in the logic of the word. Think of a "fan" as someone who's an aficionado of one thing or another, a book series, for instance. If they like it, they're a fan! "Fangirls," on the other hand, is generally derogatory, referring to fans who are generally ladies in their younger years who are rabidly devoted to whatever it is they like and aggressive towards people who don't like what they like. Are you following so far?