Ok..this is gonna sound weird! but im confused...?

ok this happened on facebook......so, on facebook, i usually add people that go to the high school and middle school in my town. last year, i added a girl that goes to my highschool ,she is about 1 or 2 years older than me..and kinda popular but i havnt ever met her..or really seen her. 2 days ago, she wrote on my picture "your so pretty <3" . i thought that was strange since i dont even know her,but i didnt rlly mind. i decided to click on her profile, and she i realised she unadded me, i didnt really care but i wondeered someone would write that on your picture to make you realize thatt. next day she blocked me. all i thought was "what did i do tto her?" can someone please tell me why someone would do that??