I was told to try and keep up my fluids, take shallow breaths, as deep breaths can accentuate the problem, and lie on your lefthand side with your legs elevated.
This is exactly what I was told when I was pregnant with Madeleine.
I felt like I was going to faint all the time when I was pregnant with her.
When I was carrying Noah... I would get like this whenever I went into my local Big W !!! Crazy !!! I guess their air conditioning wasn't right for me, because I always felt hot when I was in there.

I really feel for you... It is such an awful feeling, being on the urge of passing out.
I hope everything starts getting better for you soon
Lisa