bullying

I'm getting bullied at the moment cyber and physical. The girls are staying I'm a slut and I'm a "twat" on their social media. They said today to other friends that they think we are nasty looking girls and popularity has gone to our heads. They're pointing me out and making me paranoid thinking that I've done something.. But I've never been friends with them to do something? I'm so sad and I don't know what this looks like.. Jealousy? I don't know. Please give me tips to stand up for myself.