Be sure to have enough water with you and already IN you.
Also be sure to not have an empty tummy.
If it's the heat that's doing it, try taking a bandana to work. Get it wet and ring it out. Then wear it tied around your neck while you're on the way home. Maybe a sun hat and sun glasses, too?
I do NOT tolerate heat well, pregnant or not, and the bandana thing is a life-saver!