I have a little boy who did that for several months. He started here at 11 mos. of age and has only stopped doing this about a month ago at age 16 mos. He woke up screaming nonstop once or twice during his nap and as Daycaremamma said it was usually a poop, but if I changed him there was no way he would go back down so I would just lay him down and cover him up again and tell him to shush and sleep. It was quite a battle. Turns out the little guy was in complete pain because he had a lactose allergy that his parents didn't realize and his doctor didn't diagnose.