My daughter is 13, she had a best friend for years, recently her best friend moved on to other relationships. This really set my daughter off, we even caught her e-mailing nasty comment to other friend about this girl.
We discussed this with her and told her it was not appropriate, and we would not tolerate this behavior.
Recently however we have noticed that she constantly complains about the friends she does have, saying they are annoying, she can't stand them etc.
I beginning to think she has some relationship problems, or maybe mental.
She is hardly ever happy, constantly screams , is rude to her mother.
We didn't raise her to act like a stuck up spoiled brat, but I fear that is what she is becoming. Please help! I used to have a nice little girl, what do I do?

It sounds to me like the usual things that happens to teenagers, especially teenage girls. I really wouldn't look into it too much. She is just going through a phase and being a moody teenage girl. If it progresses any further I would be worried, but right now just let her have her space and things will work themselves out.

Puberty is what is going on here as well as the mouth stage as I call it, I feel for you as you have oh I say another 5 years of the parent knows nothing and I know it all.
As far as sending mails like she sent that you know and say were not appropriate take the computer away.. let her use it only in your presence and yes while you sit beside her making sure it is strictly to look things up for school work. ( if you see it happen again)
Phone privledges can also be restricted tell her if she cannot find decent things to say about people.
Tell her the disrespect along with the tantrums will cease as of NOW.
You have as a parent enter the hardest stage of child rearing... If you don't keep control and let it continue then little by little you will lose all control and have a daughter you won't be able to do a thing with.
This might be of some help and it does work tape her tantrums or video them and play them back to her to watch and listen too... Say this is how you look and again it will not be condoned.

Your daughter is "testing" your authority. This is tough for most parents to handle because they want their children to "like" them. It's important that your daughter respects her parents. Children do not have to "like or love" their parents but they must "respect" them. There is a big difference. Love is a higher level of feeling for another individual which we usually develop later in life for a member of the opposite sex but respect is honoring the wisdom and integrity of authority figures e.g. police officers, judges, teachers, and, most-importantly, our parents. When a teenager feels that he/she can manipulate their parents because they do not respect them, then this sets the child up for future entanglement with authority figures. Parents are not their children's friends and they are not their children's lovers which means that parents should be respected authoritarian figures for their children and not expect the child to "like" them. This, of course, doesn't mean setting irrational rules for a child or strictly disciplining a child. It means that a parent must teach their child to obey socially-acceptable rules of morality. Rules are necessary for children in the home because without rules in their home children learn that they can "get away with" anti-social behavior without consequences. Save yourself and your daughter from future problems, by setting appropriate guidelines for her behavior and then enforcing your guidelines in an acceptable manner.

TALK TO HER!?