Dizziness when standing up?

Every morning when I get up I get extremely dizzy to the point where I almost fall over. Yesterday when I got up I wasn't dizzy at all, though about ten seconds later I collapsed. It doesn't matter how long I sit up for before getting up as I still always get dizzy, my legs feel really weak and my vision blurs. I usually hold onto something for a minute before it pes, but I'd rather not experience it at all. I'm not sure if its a dietary problem or a physical one. I'm pretty fit and play sports at least twice a weak for 2 hours at a time, and I'm also anaemic.