Part of it is, in my mother's words, an age and a stage. I noticed that shortly after my daughter's 4th birthday, she got a bit... wonky, shal we say. I think that's a pretty common age to start testing their independence and pushing all the boundries. Another part is the holidays; they're contagious. My 19 month old is wound like a spring, and he has no idea that presents are coming. I think it's the general atmosphere and excitement in stores, on the tv, and in the air, and from the two big ones (who have been little hooligans for the past month!).
there's a lot of other good advice on here, so, mainly, I was just writing to say it will pass. Hang in there a bit longer, try not to bop her (or sell her on ebay LOL), and remember that they change every day. Good luck and happy holidays!