Why does my head spinn and i get blackouts whenever i get u too quickly?

whenever im lying down, or im in the bath lol, and i get up too quickly i get a total blackout-my vision goes black and cant see anything, and i hear bells ringing lol, or my head spins, and i feel like im going to faint.. i hav had this for awhile-like 2 years,, btw it only lasts for a couple of seconds