I would say taking a break, particularly during a vinyasa flow class would be sensible. I was in a power yoga class on Weds and an ashtanga class today. In both classes, the build up of internal heat was gradual at first, but all of a sudden I was almost overwhelmed by the heat and had to ease off.
Dropping into child's pose, drinking some water, catching your breath will all help gets things back under control. Learning to recognise the signs is all important. When you feel like that, take some rest, or take a more straightforward version of a pose than the stronger pose to ease the nausea.