It happened to me as well some four or five years back when I was still in Nepal. It happenned very frequently. But later on, I realised that it happened whenever I slept with my hands over my chest. When you are deeply asleep, the weight of hands make you suffocate and gives you such feelings. But you are too sleepy to realise this and in the sleep you feel helpless to move or shout. Believe me, this is true.