I don't know what to do ?

[Anne]

There is a girl in school who is always being really horrible to me and she's getting worse ! I've never been friends with her and I only really met her at the start of this year. She had a fall out with her friends and started talking to my friends and kinda joined our group. She acts all nice and whatever to them but behind their back is bitching about them then lying about it when someone confronts her which is happening quite a lot these days. She then blames me for things and keeps putting me down.
On ask fm, which until I was told she was being mean about me, I knew nothing about (like i didn't even know what it was) cause I don't have Facebook or twitter or anything anymore someone asked her what she thought about me and she was like "she's mean, nasty, horrible to everyone, full of herself, she's ugly, annoying and basically a bitch" I didn't understand this at all because before anyone asks I hardly know this girl ! I haven't done anything to her and none of what she said is remotely true! Shes also been spreading rumours that I'm a slut and telling people I've been bitching about them. She just keeps being really mean to me. I've been through something similar to this before with a girl but that was different because she was supposed to be my friend whereas this girl I hardly know !
At first I didn't want to say anything because I knew she had fallen out with her friends and everything but after thinking about it - there's not really an excuse for this ! She's doing everything to make me miserable.
I know some of my closest friends have my back and have started ignoring her and leaving her out but I don't want to do that cause it seems a bit mean ? I'd rather speak to her about it and ask why because I don't know her that well ?
The other day a girl asked her why she was being so mean to me and she said "she's mean to me" but that's not true - I haven't done anything to her ? This is really upsetting me.