Whenever i wake up or get up, i get light headed, dizzy and basically go blind for a couple seconds..why is th?

okay so its not always but its a lot or most of the time...like i get up and i suddenly feel light headed, dizzy, i start to see grey spots and basically nothing, and i shake like if i were having some epilepsy attack, like i cant control myself i fall a lot of time, and like today..i ended up in the bathroom from my bed and i was tryng yo hold on to a towel rack but i somehow fell on my but in between the toilet and shower and i slammed my head on the wall...it hurt really badly..and that's when i went back to normal..