not (oneself)
Not feeling as one normally should, either physically, mentally, emotionally, etc. I'm sorry for getting upset at you earlier, I'm just not myself today. I know Mary hasn't been herself ever since she lost her job.
not oneself
Not feeling physically or mentally well, as in I think there's something wrong; he's not himself, or She seemed to be improving last week, but she's just not feeling herself today. Also see feel like oneself.