Bitter Ex Girlfriend?

I was in a relationship with a girl that I played on a college team with. Needless to say, after things ended, she stopped speaking to me entirely. I had no contact with her for several months. She has since moved on to another relationship but still contacts me periodically. We were friends first and have a number of mutual friends, so I'm friendly with her. But she still acts very pive aggressively with me - being nice to me one minute and then saying nasty things about me to mutual friends or even posting things on facebook. The breakup was over two years ago. What is her deal and how is the best way to handle this?