Sounds like orthostatic hypotension - the body doesn't acclimate fast enough to the effects of gravity when you stand up. Your doc should be able to take your BP laying down, and then again when you stand (while watching to make sure you don't fall) - if there's a big difference, you have it. Of course, then the million dollar question is why. Make sure you mention all meds, etc. when you go.

I wouldn't mess with a head hit either! Follow your doctor's orders but... In the mean time, get up *slowly*. Sit on the edge of the bed for a few minutes and then stand for a minute or so before walking.