Nausea is something I haven't heard of with MS. It is typical of somebody with heat stroke
or heat exhaustion
, however. When somebody first gets heatstroke, or just extremely overheated, they'll get nauseous. I don't usually recommend Gatorade, as it's such a high-calorie drink, but try half-and-half Gatorade and water. It'll help rehydrate you and replenish some salts.
I live in Texas too, and on Saturday it was so hot and humid, I seriously considered moving.