Mandy does the same thing periodically, except she doesn't yelp, she does the GSOD. She often sleeps in a pretzel position. I talked to the acupuncturist, who told me she most likely gets into a position that pinches a nerve a bit, then it suddenly hurts, so she reacts like a drama queen. I usually get her to walk a couple of steps, rub her down gently and then she is okay.