Yes to postural hypotension. It could be any of the things mentioned above, but also make sure you're eating enough fat.
I went through some extreme dizzy spells about a year and a half ago, even fainting a couple of times. I did the full cardiologist workup and had no real problems. The Dr chalked it up low blood pressure and dehydration.
I went Paleo in Sept and started getting dizzy spells again. The nutrition consultant at my gym(also Paleo) told me I wasn't eating enough fat. That did the trick.
Now, if I ever start to get the lightheadedness, I think about what I've been eating, and sure enough, I've been skimping on the fat.