What Causes Intermittent Loose Stools In An Infant?

My 23 month old has had loose stools on and off over the past month. Just when I am about to take him to the dr, it hardens up. He is eating, drinking and playing normally. My daughter had the stomach bug in the middle of this month, so I just figured he caught part of that. He is having 1-3 loose, messy stools, sometimes with a little mucous a day.