Its because she's missing you now that you are gone. Thinks she made a mistake by breaking up with you. Or she wants to use you as her"seatfiller" until she meets someone new.
cause she's indecisive...and unstable. =Headcase. She realized she made a mistake, but rather than admit that to you, she's trying to subliminally let you know that she was wrong, by calling you. I mean he if you like the clingy/attention whore type...give her another shot. Just realize she comes with some extra baggage.
did you ignore her or really say no to her because ignoring leaves room for them to try harder?
... She misses you?