A girl reject my Facebook request?

[svc52]

I've known this girl for almost a month. We went to grab lunch and share drinks with each other. We even studied at the study room alone. I have class with her two times a week. We went to our school convocation twice together. She even said I'm funny (when I wasn't trying to funny and another time when I was.) One time when I sent out a group text message, she only sent text to me instead of other people. But she later, reply to the group message that took her about 8 hours. Yesterday there was early Halloween party at our school and she approached me saying "Hi (my name) how are you? I know she must of saw my friend request because she changed her profile picture. I just don't understand why.