I've had some faintness too, once on the train on the way home from work when it was really crowded & hot & no one thought to offer a pregnant woman their seat. Somehow I held on until I got to my station & felt better as soon as I got out into the fresh air and sat down to wait for my hubby to come pick me up.
The other time it happened, I was just sitting at the computer one morning, had been for a while, and suddenly I had the same feeling as on the train, really hot and dizzy and faint...this time, I went and lay down straight away and felt better.
Mentioned it to the doctor, he ordered a blood test, and it turned out that despite taking iron supplements and eating lots of red meat & vegies, I was anaemic. Since then I've been working on further increasing my iron intake (& vitamin C, which I was also already taking, to help with iron absorption) in my diet. A glass of OJ with dinner, extra red meat & vegies like beans and spinach (just as well I love red meat & just about every vegie you can name!). Better to do it this way than to just take more iron tablets. Apparently, while both meat & vegies are good sources of iron, the iron from meat is more easily absorbed by our bodies. Also, avoiding calcium-rich foods & caffeine (not having much caffeine anyway these days) too close to my major iron doses, because both inhibit iron absorption.
Not sure if it's helping yet, because the two faint spells I've had so far were a couple of weeks apart as it was, and I haven't had another blood test to check if I'm any less anaemic since the last one a few weeks ago.