I had that when pregnant with DS and the only thing that helped me was to have a big whack of sugar - when the doc tested my blood sugar levels they were really really low for a pregnant woman. Told me to keep eating every two hours, and I started drinking coke whenever I would feel a little bit faint.
Not that I'm recommending you to load up on sugar (cos you gain more weight that way..lol) I'm just saying what it was for me, cos it wasn't my blood pressure, just my blood sugar levels that made me faint...
Hope it gets better for you soon!!!!