What Causes Shooting Pain In Head?

Extreme lightning (Sharp Shooting) type aches in my head, right and back side always. Used to last just a few seconds and occurred once, max twice a year. Had an MRI done three years ago, no issue was diagnosed. As of this month (Feb 2015) the aches are back, not extreme or shooting, but last longer & continuous (same area) ~ an hour and almost daily. Please advise course of action.