E.M.
I am really surprised that your doc didn't warn you about that! When you are very pregnant, all the weight of the baby, fluid, etc presses against the small of your back. I don't know the exact part, forgive me, but I know the weight presses against either nerves or major veins and that causes light headed-ness. When you go in again, ask the doc to put a pillow under one hip so your pelvis is tilted and you are not totally flat. They can absolutely do ultrasounds and NSTs this way. It worked like a charm for me :)