Randomly feeling faint and dizzy?

I've noticed recently that when I get up and walk around, I get light headed and dizzy, and I feel like I'm going to faint. It only goes away when I sit or lie down. I've taken blood tests before, and they always have come back okay. I've been tested for anemia, and even though all tests came back negative, I've taken iron supplements. I've been off of them for a while now and have only recently been having this problem. I don't know if I should go back to the doctors, because every time I have in the past, nothing has been wrong. I've only actually fainted once, and that was after having a blood test done. Does it sound like I should go to a doctor or just wait and see if it passes? It's only been going on for about a week, and only gets bad enough that I have to stop what I'm doing like once a day.