Agro - I have the same experience with a dog - - he was sleeping and quivering and whimpering and I reached out to pet him and he almost bit my finger off ....
I made the mistake once when he was sleeing like this but quivering with kitty dreams. I put my hand on his tummy and he hissed, growled and shot up about 6 inches in the air! I came away with a claw puncture in my hand that gushed! It wasn't his fault, he probably thought something was going to get him!