My second job is a cashier so I feel ya. It's because of all the extra blood flow and progesterone. I had that happen to me quite a few times. The best advice I can give you is make sure you've eaten before your shift and try to bring a water bottle with you. My work didn't allow drinks but I hid it and honestly if they got mad I would tell them where to go.. Lol. Also it's very important your bending your legs and circling your arms to keep the blood flow moving. Otherwise the blood tends to go right to your feet and that's why you get light headed. If you have this happen on a regular basis request a chair. It's not worth getting hurt over. Hope that helps!