Mine did this too. And it was heartbreaking, because it wasn't the usual, completely indignant "I will have you all KILLED when I perfect my death ray!" cry that she normally gives during the day. It was soooooooo sad and forlorn. I snuggled her when it happened. And it stopped happening after a little while. I'm glad to read the gas theory, because when it was happening I read up and thought maybe she was having night terrors, and was advised not to do *anything* as this just prolong the terror, so I had been feeling awful because I truly, truly could not follow that advice.