What Causes Passing Of Foul Smelling Yellowish Diarrhea?

Hi, Dr.Rynne, My son is 2 years old, he is going to kindergarten. He got a cold on last thursday, it turned better on tuesday, but on monday, he began diarrhoea. It is a very strange diarrhoea, it smells like fermented cabbage, it is very yellow, it smells like calf diarrhoea. i tried diosmectite (smecta), it is clay (for medical intestinal use). It did not make him better. He has poor apetite, bad mood, he lays down on his belly very often, in every place (in the street too). It is now the fourth day. I phoned the pediatric doctor, because i could not obtain a rendez-vous before next week. She said i could give Nifuroxazide. Do you think it is safe to give 200 mg three times a day? is it Giardiasis? it looks like.