What does it mean when a ex-lover is mean?

I broke up with my girlfriend a while ago - correction, she quite suddenly broke up with me. Various issues that were out of my control, mostly lack of self-worth. We've been apart for a while and we agreed to stay friends. I'm doing a pretty good job getting over her... but the friends thing has eroded. She is overtly mean, sarcastic and accusative whenever we interact, which is relatively often. What can this mean?