I have not heard of this either, but my pregnancy years are WAY behind me now. If your doctor says this is normal, I would call back and ask WHY?
My experience with passing out though, is different. I passed out nearly everyday for over a year, following strokes, so I do have some suggestions for what to do when you get that feeling.
At the first sign, sit down immediately, and protect your head. The major problems with passing out is the potential for injury when you fall. I used to slide myself down to the floor, using the wall or any sturdy structure. Do not spend time trying to talk, or be polite. You usually only have a few seconds before you faint.
Passing out causes your brain to reset itself, so do not get up until you are good and ready. Put the potential embarrassment aside, and avoid that gut reaction to pop back up like nothing happened.
I would ask your doctor how long this "normally" happens, and if it goes beyond that point, insist to be seen. Keep yourself well hydrated, rested and relaxed, as much as possible. Pregnancy brings many changes to a woman, but the secret is to take these in stride, and enjoy the miracle of motherhood.
I wish you the best...