I also experienced a fainting spell on Sunday. Suddenly, felt very giddy, broke out in cold sweat and felt like my feet was going to give way. Then on Monday, I had a similar experience but a very mild version.
I went to the gynea to have it checked out. Everything was ok. Blood pressure was normal too. According to gynea, when we are pregnant, the blood flows to many directions, particularly the womb. Sometimes, we could be sensitive and felt giddy. So he advised me to sit down whenever I feel giddy and then put my head between my legs so that the heart is at a position where it is higher than the head. After a while, it will be back to normal. He also gave me some time to rest as he said it could also be caused by stress. After resting for 2 days, I felt a lot better and so far did not experience the fainting spell again.
FYI, I am also not on any specific iron pills. Just the normal multi-vitamins.