My daughter went through that last year. She ended up having a urinary reflux issue--which we wouldn't have know without this happening. But the main reason that it was happening was because she was constipated! It has pretty much subsided..but she does still have trouble sometimes and thats when I know to mix her laxative in her juice... These things can be very embarassing for them to talk about -- my daughter wouldn't use pull ups and I wasnt going to force her to do something so embarrassing. We used Serenity pads. She was so embarrassed and it wasnt anything that she could control.
Just wanted to chime in, in case the issue wasn't emotional. Im not sure if boys can have the same thing happen.