She doesn't want to spend 20 seconds explaining that she's not attracted to you anymore, so instead she will spontaneously start to act like a complete douche bag. This will continue for the next 2 months as she is hoping you will eventually catch on and dump her. If you don't, she'll become totally ruthless and will then do everything she can to cause you pain.
This is because she's a really sweet person who doesn't want to hurt your feelings.
This is because she's a really sweet person who doesn't want to hurt your feelings.