Yep this happened to me at work in October and my doc put me on leave early (I work at Bunnings, concrete floors, 9 hour shifts and no airconditioning - bad combo ) but same thing as the other ladies have suggested. Mine turned out to be low blood pressure and I was dehydrated, I find I'm heaps better now but I do drink 2.5 litres of water a day now so I don't feel 'funny' anymore. Hope you find out soon what going on!