Why do females act the way they act?

I use to be very good friends with this girl at my school until one day i moved up from jv cheerleading to varsity cheerleading and i took her spot.Now she so calls cant stand me and she always sayin im ugly and she use to always be thirsty to hang out with me.And everytime i see her see just stares and now she haves other people not talking to me anymore.What is her problem? why is she acting like this