It could be hormones, even though she hasn't gotten her period or developed many secondary sexual characteristics. In fact, if other girls are more developed, she may be feeling some pressure (from them or from general society) that she's "delayed" in some way.
Also, social dynamics change a lot at this age. Kids are in and out of friendships, distracted by older kids and the lure of "going out" and so on, so she may be experiencing some feelings of "betrayal" by friends and that's ticking her off. Is her class very involved in cliques? Sometimes, at this age, kids evolve and mature, and they find that their old group isn't really their cup of tea, but they are nervous about moving beyond to other groups. So loyalties ebb and flow, and that's stressful. She may feel secure with those she has known longer, but also feel they aren't all that aligned with her personality or interests anymore. Hence, the conflict.
I think she needs to find something that appeals to her skills and interests, and that mix her with different groups of kids. That helps her find an area of expertise and achievement, and also keeps her from being with just one group of friends all the time.
I think it's wise to slowly and gently talk about women and teens supporting each other and allowing for differences. Putting each other down and being rude just doesn't fly anymore. With all the talk (in the news, in schools, etc.) about not listening to women and girls, it's a good time to say, in words that make sense for you and your daughter, "Look, you don't want to be pushed around and you don't want to be abused. So you can't disrespect other females yourself."
I also think you should be alert to possible early signs of depression. Whether that is going on or needs to be addressed is up to you and perhaps a doctor or counselor or school psychologist, but just be open to it. You might call the school psychologist and state your concerns, and have that professional keep an eye out to see what's going on during the school day. That will give you a better perspective on what's going on when you aren't there.