Why does she hate on me so much?

I'm in a tech school, and my baby's mother who knows I was going to this school ends up enrolling in my cl. I did have with her sister, but that was when our relationship was over. We have not been together for about 2 1/2 years. At first it was all cool, we had no problems. I never talked bad about her(not that she doesn't have dirt either). But one day she just starts slandering my name, saying I'm a bad father, calling me horrendous names, even so far as to call me a rapist. Honestly, my life has been so wonderful being seperated from her. What would cause her to say such hurtful things? Is it 'cause my life is going to so good and her's is so awful? I walk in with a smile everyday at school, then all of a sudden here she is slandering. Someone explain to me wtf is her problem?