Easy, easy, easy, easy, EASY! This is just to easy to address right here. If you have ever read Derek Rake's stuff, then you would know EXACTLY how to respond to this.
To respond, the most important thing is to be nice and respectful about it. You don't want to come off as a d1ck or angry about the fact that she is being a bitch to you. Here is the common response you should say:
"Strange. You looked like a nice person from a distance. I guess looks can be deceiving. Oh well, have a nice day then."
Here is another potential answer to rip off:
"You know, I sort of had the feeling that you were not the nice type. However, I decided to give you the benefit of the doubt and not judge a book by its cover. Oh well, have a nice day then."
By saying this and saying it respectfully, you are initiating a challenge with her. A challenge to try and prove you wrong. Unless she doesn't care about you, she is going to try and approach you after that. She will apologize and insist that she is a wonderful person. The key here is to not make it easy on her. Ask her something like "so you mean to tell me that you aren't this snotty usually?" Try to make it hard and a challenge for her to get at you.