Ugh that sounds frustrating. W-E-L-L, I'm not great with advice, but it sounds like she might possibly be jealous of you? Is she mean to your other friends too, or did they agree that she was being mean to you? Oh and did you just start middle school?

Front. Sorry I know it's annoying when people do this, but it was stuck way back there.

Ouch. The thing I would do is ask her, actually probably by email or letter or a note, if you offended her in any way and apologizing if you did. Then try to be her friend as much as you can - stand up for her, help her with anything she has problems with. Also, make sure that you don't do things that make her feel left out or like you're above her - I have seen that hurt friendships before. Assume that you're the problem, not her, because that will do more toward mending the problem, while thinking she's the one wrong will do nothing but hurt it. If nothing works, though, accept that relationships change over time and that friends change, and occasionally, painful as it is, we actually grow out of friendships. Keep her as your friend as much as you can, but things change. You may just need to deal with it and make other friends while keeping what you can of her. Let us know what goes on, too - we are your friends, even though we're removed from you a step, and we care about you.

-EH

Erm... Sorry, I haven't had enough friends in my life to really have advice.

Hehe, I kinda have lots of experience with that sorta thing.  I think that almost all of my friends have done that to me at some point in within the past two or so years! :)
Possibility 1. A lot of times, girls who act like jerks, randomly kick out friends who they've known for years, and only like the popular girls have more issues than you might know about.  Such as, the girl who I'm thinking about lived in an abusive home with alcoholic parents, mean siblings, and just a really rough life.  Not being a Christian, the only time she feels important, etc. is when there's lots of dramadramadrama.  So, whenever there's a slow in the drama at home/school (such as, after her parents got divorced), she decides to create some of her own drama (such as being a jerk to her friends), just to have people thinking about her, even if they're not good thoughts.  Soon, something will happen in her home again, and she'll go back to her friends and ask for forgivness (or just act like nothing happened in the first place).  You can either play along with her drama games, tell her drama's stupid, or tell her about Christ and bring her to church (if you're a Christian). :)
Possibility 2. I'm not going to get into an awkward talk, so this is all I'll say.  Perhaps puberty is killing her common sense?
Possibility 3. And maybe she just feels pressured to be popular.  Liiiike, at my camp, I used to be bestest friends with this one girl. Then, the third or something year, I went to meet her at the sign-in place, and when I said hi, she just stared at me like, "Ummm, yeah, what do you want??"  I said bye, left, and, as soon as I did, she and two other "cool" girls started cracking up laughing.  She didn't say a single word to me for the rest of the week.  Really, I don't think "cool" can be used to describe "popular."  What makes them cool??  Their bratty attitudes??  How often they insult people??  ETC????  Eeehhh... :P

Anyway, you can try your best to keep on loving her as a friend and hope she'll eventually snap out of it, or try to make new friends.  I know, that's what lots of people say in advice, and then ya go, "hey, you know, making friends isn't as easy as it sounds!!" and you're right, it isn't.  Still, it's better than being "friends" with a brat. :P