Our dog had this shaking/trembling problem with her head when we first got her, which seemed to disappear. Just recently, it seems to have come back, but a little worse: she twitches. She has been doing that and has been losing her balance (her legs just go out from under her as if she were slipping on ice) as well all this morning so far (or at least since around 8:00 AM). If she doesn't lose her balance she'll sort of put her back legs closer together (I guess to prevent her from losing her balance) and walk slowly. It seems to be over now, as she's relaxed and going to sleep. My mom thinks it might be something neurological. Whether it's serious or serious yet or not, I don't know.