Heard sugar & simple carbs turn into same thing once digested. Why then do I feel sick after eating much sugar, but not after eating much wheat/rice?

It could be. the rate at which these substances are metabolized. Once in your digestive tract sugar hydrolyzes into fructose and glucose. Glucose starts out as a carbohydrate but is quickly stored as a fat. Rice and wheat are made up of more complex chains of carbohydrates with fewer simple sugars. The effect of these foods on raising blood sugar may not be as rapid as with simple sugar. Hope this helps:)