Recurrent sickness

My 4 year old got a fever and had pink eye in both eyes with a right ear infection, he also had watery diarrhea accompanied with stomach pain. He started feeling better then my 2 year old got it, he ended up with the diarrhea, vomiting and fever which went away and came back. Now my 4 year old is throwing up again. What is going on?! Doctor said possible stomach flu but didnt do anything further and when you get it and it goes away it's not typical to get it immediately again. ER doesnt want them coming in unless fever is above 104 and they're not drinking. Should I be concerned about the reoccurrence?