Most Helpful Girl
Someone who acts like this likely has some deeply rooted issues of insecurity and wanting attention. They might crave the attention of someone and as soon as they lose it, they freak out. It's also a sign of selfishness. But you have to realize it's abnormal behavior and she likely as I said, has some very serious issues she is struggling with and that is affecting her behavior.
Most Helpful Guy
Because some women are fucking retarded in the head.
She's just pissed that you found someone else before she did. You're supposed to be miserable. You're not supposed to have moved on. The fact that you have bothers her because it means she a) has no perceived power over you, and b) is alone.