Alright so I googled around and I don't think this is a hypnic jerk.

Some nights, I would be woken up out of nowhere, without warning or reason, to a sudden shake of my bed. It feels exactly as if someone took their hand and SMACKED my mattress at the foot of my bed as hard as they could. This happens about once a week.

Anyone else get this? Does it have something to do with sleep paralysis/REM?