I fainted twice when I was pg, the first time I was around 17 wks and apparently its very common to faint around that time. The second time I was more like 30 wks and it was because I was low in iron.
It must be terrible to be pg in that heat! Hope youre feeling ok, make sure you rest up and drink lots of water. I would prob mention it to your midwife if youre concerned.
All the best