4 year old, been potty trained for 2 years, dry at night, now pooing in pants!?

help!!! what do i do, the other day he got home from preschool and he had done a poo in his pants, he couldnt explain why he did it, didnt make much attention to it. and again he has just done it!!? anyone been through the same?? or is it just a phase? :? :?