That. Is standard, text book sleep paralysis. Mostly, it's included the feeling of being held down, but everything you describe is standard experience. Basically, your sleeping mind and your waking mind have locked up and while you are somewhat observing the waking world, the part of your brain that controls movement hasn't basically turned back on yet. So your subconscious mind reads this as "I'm trapped. What's holding me down? Who's trying to keep me here?" and it fills in the blanks as best it can.