It sounds as though it is related to blood pressure. If it happens when you stand up or move quickly from one position, its a change in you bp. Just take your time when standing up. I get it sometimes when I get out of bed, so instead of just getting straight up I sit on the edge of the bed for about 30 secs. My BP was fine though when i had an ob appointment last friday. If it worries you, go and get it checked out.