18m old.diarrhea for over a week and now diarrhea and frequent vomiting 2nd week.stool,blood,and urine sample came back fine. what could be the cause?

Sometimes: after a significant bowel infection, there is a temporary lactose intolerance caused by damaging the production of lactase enzyme in the bowel. In order to stop this cycle, I would eliminate all milk and dairy products for 5 days. Give BRATTY diet....bananas,rice, fresh apples or applesauce,tea or clear fluids (no apple juice), toast (cheerios, dry pasta), and vanilla or lemon yogurt.

See pediatrician for: re-evaluation for weight loss, dehydration, & overall health & development. Giardiasis, a parasitic intestinal infection, can last 2-4 weeks. It's diagnosed by stool exam for ova & parasites or stool antigen for Giardia. A stool sample to test for blood is also needed. Excessive juice intake can cause diarrhea, rarely vomiting. Lactose intolerance & food allergies are other possible causes.