It has happened to me 2 or 3 times, but it's been a long time. At first I had no idea what it was and it scared me to death. A few years later the Awake! had an article on narcolepsy, and sleep paralysis is aparrently is one of the symptoms.
(Awake 4-8-91 p19,20)
"Other symptoms often include sudden muscle weakness, sleep paralysis, and frightening hallucinations."
"REM sleep features may even intrude on a patient’s waking thoughts and superimpose a vivid dream—or a terrifying nightmare—on reality. He may awaken in bed, his body paralyzed in the REM state, and hear voices and see frightening things. Such waking dreams (called hypnagogic hallucinations) may also occur during the day, and about half of all narcoleptics experience them."
I was relieved after I read that.