2 year old had bad gastro flu a month ago, poor appetite since, stomach pains, now diarrhea again, headaches and low grade temp a month later ! why?

Lots of causes: Infection, malabsorption, lactose intolerance, there is even an entity called toddler's diarrhea. Until you get to a doctor and get it sorted out, avoid sweetened beverages like juice and sports drinks. Avoid foods with high fructose corn syrup. They act as laxatives. Fiber supplements can bulk up the stool. Increase fat in diet with items like whole milk or peanut butter can decrease diarrhea.