Ah, the joys of our little girls turning into tweens! First, please know you are not alone. I'm amazed to this day my parents didn't send me to a nunnery when I was a teen!
My 13 y.o. daughter tends to save up all her frustrations and anger for stuff that happens outside the household and share it all with us. In a negative manner. In other words, we're her favorite whipping post. Broke up with the BFF? Yes, it'd be logical to talk to mom about it. Instead I'm going to make little brother pay the price, because he's an easy target and at the end of the day i know he'll forgive me.
I would encourage 2 things:
1) Keep an ongoing open dialog with DD. Ask her about the friends, boys, teachers, classes. And then listen. And then ask more questions. Refrain from judging or offering advise unless you are SURE she wants it. (hint: most teens don't want your advise. its your job as a parent to imbide the advise without them knowing it. i use a lot of leading questions to subtly get my point across.)
2) Review her television viewing habits very, very closely. One of my friends is a teacher attended a seminar about what's ruining today's teens. Top of the list? "Sarcastic comedies" on television. You know - the ICarly's, Victorious, Family Guy, Always Sunny, 2 1/2 men, Letterman, Aquateen, Office, etc. Heck, throw Sponge Bob into the mix. The problem with all of them? They are filled with rude, sarcastic, poorly behaved characters. The audiences laugh at all their antics. And by the end of the 30 minute show, they'll have paid no real price or consequence for their rude or poor behavior. And the next week comes and there are still no ramnifications for their behavior. Sounds familiar, right? And if your tween is anything like mine, my kids watched a couple of these shows (the Nick ones.) GET THESE SHOWS AND OTHERS LIKE THEM OUT OF YOUR TWEEN'S VIEWING SCHEDULE. We pulled the plug on all of these and started watching more Animal Planet, Discovery, Science, and History channel. We were able to find shows they like and we have noticed a huge change in the behavior.
The changes will not happen over night though. In most cases, kids have been raised on a few of these shows and the behaviors have been learned and role modeled that they are engrained. A good discussion about why they are bad and helping your tween understand just how negatively they affect her would be a great start.
Good luck!