What does it mean where your about to fall asleep and you Spazz awake.?

Sometimes, right before I go to sleep, I will be just closing my eyes and ... WHAM. I'm awake. It feels like someone got on top of me, put there hand on both of my shoulders and shook me so hard. The first time this happened I thought I heard voices afterwards, but it was just the blood in my ears. What do I do, this doesn't happen very often but it's very scary when it does. Should I go to a doctor, or maybe is it a growing thing (15, boy)?