A 20 year old man presents at the medical OPD in a hospital with a complaint of abdominal discomfort, diarrhea and muscle cramps . From the case history, it appears that he had consumed a large quantity of milk and other diary products for 3 days on a festive occasion. The house -surgeon who examined him , (after ruling out other causes of acute abdomen )advised him to avoid milk and milk products. The patient followed his advice and became normal in 2 days . What is your diagnosis ?