My dd w/b 11 in a few weeks, and the last month or so I am also seeing a change, like she is snappy for the littlest reason. It is hard not to blame myself for a sec, like, "what did I do wrong as a parent, is this how she is going to be" kind of thing, but I have learned that allowing that kind of fear in only worsens things.
To read your post actually made me feel better because it reminds me that she is standing upon a threshold, and it is scary for them. Even if they aren't aware of it. Did you read that article in Mothering about the ritual for their menses? My dd has been very interested in that. I am finding that keeping an ongoing dialogue with her about her behavior, and how she is reacting to things around her is important and effective. The theme of our last big talk was not trying to change who she is, but how to help her be her authentic self.
My DH and I came up with that when we realized she is sort of trying out different attitudes, and that the communication is sort of breaking down. We gave her examples of people she knows who are communicative and who are mumblers with nothing more than a shrug to answer back with. It was good, and we keep referring back to it when she starts to snap, or shrug or pout. Just reminding her to find her voice and communicate the feeling she is having.
I know I am rambling. I will stop here. I hope it becomes easier. Feel free to email me anytime.