Or maybe shes really interested in your friend and you just happened to be in the way. By the way i'm talking about a specific girl, you do not know her so what i post might not apply to you.

very good question. i also ask such questions on occasion.
Answer: you are probably not misinterpreting the signals; she is probably teasing you. she might be insecure and trying to make up for it be acting really slutty when she is really not. Or she might have the ashawo sydrome, she wants to phyuk and she wants money but she doesnt want you to think shes a slut. and then there are times when its just you, maybe you just suck at reading body language.