This has happened to me a few times before. I'll lay down, not trying to LD, just normal sleep like usual, then bam... suddenly I get this sick feeling in me, a really sick one. I feel like somethings wrong, I don't have anything to be afraid of, just feeling fear at literally nothing. If I open my eyes, it goes away almost instantly, even though in a dark room I still only see black.
So, recently, it happened again, but this time I keep my eyes closed and remind myself there's nothing there. It just fades away eventually... and suddenly it's morning.

Anybody know anything about this? or experienced it?