My five year old boy is totally potty trained. He started school in Sep and seems fine with it all, and is outwardly a settled and a confident little chap. Recently, though, he's started wetting the bed at nights. We make sure he's had a long pee before he settles, but it's been three nights out of the last five now. Could it be anything medical? Or might it be psychological? Should we be worried or just tell him to try harder?