How To Treat Diarrhea,constipation And Stomach Pain In 2 Year Old?

My daughter is 2 years old and i had her to her pediatrician today and he said she just has a viral infection she has had the diarrhea since march 8th or a few days before and i had her to the ER and they did an xray it came back normal and she had bloodwork and that came back normal as well, she has diarrhea one day and then constipation and she is crying like it hurts her stomach, she had piloric stenosis at just 2 months old and had surgery to correct it but i keep thinking its something more serious then a viral infection, her ped also said she might be sensitive to lactose so cut down on her milk, this was the same dr that didnt run any test to help me when she was vomiting up her entire bottle when she had the piloric stenosis.