Here's the thing, you haven't talked to her in ages. Doesnt' sound like you did anything.
So my best guess is there could be way more going on in her life. A boyfriend that's jealous, her own self doubt, anything.
If you still have her email, send her an email.
Tell her that you were happy that you had the chance to say hi, you noticed the facebook thing and hope that if there were anything you did to piss her off, that she would talk to you.
Wish her well and try not to think about it.