In Which I Pass Out, and Much Follows

Long story shortened:

I stood up out of bed one morning and more or less fainted, then had what might have been a seizure, and so I called doctors and we did a ton of tests, one of which was the most unpleasant thing I've been through in years.
(If you are incredibly sensitive to stimuli of any kind, never allow someone to flash an industrial strobe in your face at increasing intensities while you are effectively immobilized on a bed. You will cry, and then the Tech doing the test will feel horrible.)

So now that all the blood work has come back "healthy", I'm still waiting on the brain scan results, and the doctors have no idea what happened yet and will probably be writing it off as "just something that happens when you pass out" without ever telling me how to avoid passing out, or explaining why, since that day, do I keep getting dizzy Every time I stand up instead of just occasionally.

Argh.

For the last time, yes I am drinking Plenty of Water.

Update: Bloodwork is fine. EEG is fine. I have a (relatively) normal brain- not a seizure, yay! I have great healthy blood pressure, I am not anemic, and I am not diabetic. That pretty much rules out possible likely reasons for nearly fainting whenever I stand up too fast.
I am thankful it is none of these things, but I'd sure like them to figure out how to keep me from being a Swooning fragile flower the rest of my life. They have no idea why I get dizzy and then faint or nearly faint. That could kind of hamper some of my plans for an active lifestyle.