I Have A 3-yr-old With Diarrhea For 5 Or 6

I have a 3-yr-old with diarrhea for 5 or 6 days now, not acting sick otherwise. No fever, no abdominal pain, no vomiting, not acting sick - still playing and having energy and usual appetite. Then yesterday (day 5 I think) he was acting tired, lying on the floor at times to play instead of running around, but I think he was also worn out from the excitement of having family in town visiting. But I think he might also have been getting a bit depleted from the diarrhea. It is watery and alot comes out at once and is very strong-smelling. There has been some gas as well. The diarrhea isn't an all-day-on-the-toilet situation; but when he goes, it's diarrhea. Now he has a slight rash on his belly - very light pink, not severe, not itchy. What could be causing his symptoms?