I actually have a similar story with a girl best friend and she an I were super close. Then one day she just starts talking about how lonely she is and how she wanted a bf and before I could say anything to reply she asked if I would be her bf. I awkwardly said yes and for about 2 days it was great then she started acting distant and we started fighting over some stuff. I said I wished it was back to when we were friends because I was happier that way and I broke up with her. Then she really started making things bad she started rumours at school about me and turned my friends against me. She made my life miserable. It got to the point where I got the cops involved and I got a restraining order against her and I ended up changing schools to start over.