Does anyone know why i feel the way i do when i stand up?

mostly every time i stand up, from siting or laying down anything even if it was just for a minute when i get up my vision to see blackens like a fuzzy black but i can still sort of see, i go lightheaded, and my body feels really heavy the feeling only lasts for like 5secs or so but still does anyone know why this happens to me? This has been happening to me for years now but its really starting to effect me now.