I have the same problem as you, and have had it for many years already. Many times I get really light-headed when I stand up, and I start to feel like I'm about to faint. Oh, and my mom has high blood pressure
as well-- maybe that has something to do with it?
I suggest visiting the doctor, as I am no expert at this.
Perhaps I should make a visit as well. haha.
Sorry I'm not of much help.