Is this girl really mean and rude or is it just my imagination?

It's not your imagination.. She's probably just insecure and needs to make herself feel better and does that by talking about you.. something must be wrong in her life that's the most likely reason. Honestly, if I were you, I would just laugh at her every time she walks by because obviously there's something you're doing to make her "hate" on you . So when she sees it doesn't hurt you maybe she'll get over herself.. and grow up