Did the whole thing happen in a split second? If so, it was a hypagogic jerk - they happen sometimes when you become extremely relaxed, almost on the verge of falling asleep. They're often accompanied by a very sudden falling sensation, almost as if you tripped within an instant, and then a jolt. Often they wake you right up.

If it was just a gradual falling sensation, then that definitely sounds like the onset of SP, especially if it was accompanied by the REM (rapid eye movement) and fast heartbeat.