White Flashes

for a few years now ive periodically experienced white flashes.

sometimes these white flashes are accompanied by dizziness afterwards.the white flashes only last probably less then a minute and then my vision comes back to me.when it occurs though, everything goes white and i dont see anything but the white. i guess i never thought too much of it.they always happen after ive been lying down and when get up.i thought perhaps i just got up too fast. i went to a doctor when this first happened to me, probably 2 or 3 years ago(i am now 19 years old). She didnt do much examining or anything,just simply asked me a few questions and said it was probably due to dehydration?

i was wondering what causes these white flashes and if its more then simply just being dehydrated.