lightheaded on standing up

In the last week or so, I keep having episodes of being *very* lightheaded/dizzy when standing up, particularly after I have been sitting for awhile. I'm almost 45, run regularly, do yoga, at my normal weight, eat pretty well. It's dry and dusty where I am and so my nasal passages are pretty dry. I've tried to make sure I drink more water, but even drinking about 1.5 liters a day, it still happens.

Any ideas what might be causing this?