I was sleeping and then my whole body jerked suddenly and woke me up, what is it? Am 20yr old female, it's happened only occasionally over years.

Very Common . There are hallucinations i.E suddenly falling off the edge of something, more so upon sleeping than upon waking up. They are common with physical fatigue. Perfectly normal and happens to everyone of us on a regualr basis. Do not worry about this. They happen to me too.
Nothng. It happens. It's of no concern if it's rare. It could just be a brief electrolyte imbalance, a dream, or your body needing a gasp of air.