I know exactly what you're going through. I used to have it happen to me all the time myself. It's scarry as hell until you understand what it is. There's a mechanism in the brain that renders us immobile so we don't get up and act out our dreams. I've even seen video of a cat where this mechanism was severed. The cat was up hissing and scratching with it's eyes shut...sound asleep.

When it gets freaky is sometimes you're at the threshhold of sleep and awake. You can have your eyes open, but you're half asleep, and you can literally dream things into the room. The first time this happened to me I thought demons had me pinned down, and I was hearing all kinds of wild noises. Another time a giant bat flew into my room and I was immobile and couldn't fight it off. I wasn't going out without a fight and all I could do was spit, so I was ferociously hockin' loogies at the bat! I finally came to with slobber all over my chest...lol

Usually this happens in adolescence when you're going through a lot of brain chemistry changes. It can be caused by stress or fever too. It's nothing to worry about any more than a nightmare is, so relax. You'll know what's going on next time and it won't be so scarry. Ever wonder why it's so hard to remember dreams? It's a chemical reaction in our brains on waking so that we're capable of distinguishing reality from a dream. Our physiology is looking out for us, but sometimes it doesn't function as properly as it should.

It is interesting to note that in most of the ghost alien abduction stories I've read, the situation was ripe for, or identical to sleep paralysis. It's a phenomenon that's probably responsible for a lot of our cultural mythology.

So you're not going crazy and you're not being attacked by spooks. If it's a real problem you might try changing up your sleeping habits and avoid caffeine n' such before bedtime....hope this helps..