This girl keeps pretending to be my friend? What do I do?

[Mandi]

I know saying this makes me sound conceited but I'm fairly popular at my school and get along with everyone which is why I think this girl is doing this. Anyway, there's this one girl who keeps writing things on my facebook wall like "we should hangout soon <3!!!!" or she'll comment on my photos and compliment me. When I'm with my friends she'll come up to us and act like we're best friends. But alone she's a total bitch to me! She calls me names and makes fun of the things I wear and calls me stupid... I don't know what to do! Everyone thinks she's sweet and nice but only my best friend hates her and knows what she's like. Whenever I tell someone else they'll be like "you're overreacting" or "I think she was just kidding". What do I do!!!?