I don't know if this is normal or not- please let me know if this happens to you.

I'll be in bed about to fall asleep, in that stage between sleep and waking. Suddenly, I'll just POP awake, extremely startled, as if I was catching myself about to fall. I can never remember what it is that startled me, but I'm always surprised to find myself flat on my back, safe in my bed before I wake up a little and come to my senses. It doesn't do any harm (other than give me a good jolt), but it's terribly uncomfortable.
What is this called, and has anyone experienced it?
Thanks,
Theresa