I have experienced being held down and suffocated by some unseen physical force while asleep. I am certain to this day that the force was spiritual in nature. As a few friends of mine have experienced the same, I figure we must have been smoking the same bad stuff at the time...
This is similar to
Night terrors some children experience(myself included). Science has recently determined that the part of your brain that controls breathing actually falls asleep and the feeling of suffocation is exactly that. Your conscious mind wakes up and has to wake up that part of the brain that controls breathing. The brain usually learns that this is a bad thing to do and they outgrow it. Perhaps something you were doing caused your brain to forget and get sleepy. <img src="/ubbthreads/images/graemlins/ooo.gif" alt="" />
Sleep Terrors 
