Hot Yoga 101 and rule #1 when starting off, trust your body.
Your body knows. The instructor doesn't. Heck, they may even bump up the temperature for kicks (or by mistake). How will you know that you didn't have enough water and the class isn't pushing your body to the edge? Well you DO know.
Trust the effects you feel, the feelings, the physical changes. The point of yoga isn't to come out first, if you want that go play football. Sit out if you have to, take a break, rest, relax, after all, you're there to improve, not to kill yourself.