hi,
Any intestinal injury be it a viral,bacterial infection or a food allergy causes a transient lactose intolerance.It generally takes a month for the intestinal cells to regenerate and the intestinal lactase levels to increase.They call this as post infectious lactose intolerance.The bacteria, virus may have gotten away in the first few days so the test results may not be positive.Also any diarrhoea generally seems to be more after intake of liquids and after eating.So may be u can wait it out for say 5 days more,but keep a close watch on her hydration,urine output,level of her fontanelle and activity.If any cause for concern then u can always revert back to your ped.