Why do I feel dizzy when I stand up?

My job requires me to be bent over for 3 hours flipping boxes but when I go to take a break and stand up I get really light headed and feel extremely dizzy and sometimes I feel like I am going to fall over. It last for about 10 seconds and then I'm fine. I drink plenty of fluids, a mix between gatorade and water and I work out everyday. Any ideas on why this may be happening to me?