Long bike ride in the heat. Immediately after had diarrhea, vomiting & nausea. Better 6 hours later after nap/Gatorade. Heat exhaustion or norovirus?

Overheating. Almost certainly the exercise and heat. This is a bit like asking whether a sudden headache is from clubbed on the head with a baseball bat or a brain tumor instead. Such a coincidence could occur but obviously is unlikely. Unless you have been definitely exposed to someone with viral gastroenteritis, it's probably not the problem. And norovirus wouldn't improve so fast with fluids and rest.