What Causes Recurring Bouts Of Frequent Bowel Movements?

My 7 year old, who has been potty trained since 3 years old has been pooping his pants a lot lately. He will go a few weeks with no accidents and then we are on day 4 of pooping his pants every day. He will spend all day at school with dried poop in his pants. I can't tell if he really doesn't know its there or if he is just embarrassed. On the days that he has accidents I notice he will run to the bathroom several (4-5) times to poop. It is not diarrhea, and it doesn't appear to be hard as if he were constipated. Does this sound like a behavior problem or something more like lactose intolerance?