Why Am I Feeling A Shock Wave Through The Back Of Head?

Im a 59 yr old female. most nights when im trying to get to sleep and just before I drop off I get a shock wave through the back of my head this happenens most nights its not painfull and only lasts a couple of seconds. sometimes the initial large one is followed by a couple of small ones. Its making me nervous about going to bed because I knoe im going to experience this before I can go to sleep.