Is this normal, or is this weird?

This happens to me everytime I sit or lay down for a long period of time like and hour or so without moving. When I get up from sitting/laying for a long time I get really of balance and wobble around and my my sight goes black and then everything comes back normal in like 3 seconds. I don't know if this is suppoased to happen and it freaks me out. It just happens for like 3 seconds and then everything is normal again and it only happens when i get up and start walking after sitting or laying. Im not sugesting its a health problem or anything I just want to know if other people have this or if its not normal or what it is.