It's more weird/disorienting than awful. If you've ever been walking, tripped, and then caught yourself before you fell, it's like that. The difference is you're in bed, so you're half asleep, and you can't fall because you're already lying down.
I see, thanks for the explainer. Guessing based on the comments it must be somewhat common for this sensation to happen to people at least once in their life. The only time I had an odd sensation in bed is when I actually fell out of my bed, it was very disorienting but the difference is I actually fell out of my bed.
It's such a weird feeling. I can't sleep on my back so whenever I would forcefully try myself to sleep in that position that would happen to me. It's always happen when I'm just about to fall asleep.
It's never been super bad experience for me, more annoying because, well I'd like to sleep lol. For me it's basically the same feeling when you're sitting and nod off and catch yourself before you tip over/fall. Usually accompanied by a sharp inhale and an "oh shit!... oh anyways, sleep".