maybe because she feels like u 2 have nothing to talk about since u guys broke up. maybe she still has feelings for u and doesn't want to feel hurt every time she sees u? i don't know I'm just guessing here.
Has she been friends with you since the break up? If not she may think that ex's aren't supposed to talk or she simply doesn't like being around them. If ya'll have been friends since the break up, I would need more details on whats been happening.