Why is she like this?

There's this girl in my homeroom, and she is friends with 2 of my friends. She has some sort of problem with me and i don't know why. I never did anything to her. She was on my field hockey team, and she never made any effort to be nice to me. For example, it was my friend's birthday and she made a card for my friend, and everyone signed it. The birthday girl let me sign it during math. On facebook, the birthday girl put a picture of thcard up, thanking everyone who signed it. The girl in my HR commented on the picture and said stuff like "Ew why did she sign the card? I don't want her grossing it up. She can hate me, I don't care." It legit made me cry. My mom said she's really insecure. But why is she like this?? Sorry for the novel btw.