It could be several things causing this. From Congestive Heart Failure, to possible dehydration. It could also be high blood pressure. It's impossible to say for sure without a physical examination and blood work to get to the cause.
My best advice is to cut down on salt, elevate your legs, drink more water, and when you are standing from a sitting or lying position go real slow to cut down on the vertigo. In the mean time, make an appointment for a physical.