My 19 month old has been vomiting for about 3 months. Sometimes it's once a week other times it's 5 days in a row. When he vomits he empties his stomach of all food and stomach fluids. There is not pattern, sometimes it's when he is eating, sometimes in the middle of the night, sometimes 3 hours after eating. There is alot of thick, chunky milk in with the food and liquids. What could be causing this?