sharp pain in the back of head? please help.?

[hithere:)]

so about 5 or 6 months ago my stepdad had tipped me over and i hit my head on my bed. it hurt, but no more than any other bumping your head incident. so anyways, i kept getting this sharp pain in the very back of my head. itd last about three to four seconds and it was excrutiatingly painful to the point of i'd get dizzy and id have to gain my balance again. So i went to the doctor, got an MRI, they didnt see anything. after that, it kept going for about a week or so and then stopped. so i thought it was done, but over the last week(its been about 4 months since it last happened) its been happening twice as much as it did before and it hurts almost a bit more. idk what wrong, and idk what to do? the MRI results were completelty normal. btw im only 16, if that would affect anything,