Doesn't seem to me like you did anything wrong, OP. My advice (this coming from a girl) is to hang around your guy friends more. Possibly explain the whole thing to your sister, since she may be more likely to actually listen to your side of the story.

Also, consider telling her current boyfriend about this. If she's that much of a skank to say something like that, he deserves to know.

Now that I think about it, though, she may be having trouble getting over you. You know how in elementary school, you act mean towards the person you like? Same kind of thing, except worse.

Unless she is just a manipulative, crazy bitch, in which case RUN LIKE HELL.