2 year old throwing up every other night...?

My son has been randomly throwing up at night, during the day he is fine happy, and seems healthy except the once a day diarehha. When he throws up its comes back up like his tummy was storing his milk and curdled it then sent it back out, He is not lactose intolerant. And my Dr. said to come back in a week if he's the same. Anyone have any ideas or tips??