Started wetting the bed

My DS who is 3 has been potty trained for over a year and took to it really well, he has only had two accidents in this time, and has never wet the bed..UNTIL NOW. Over the past week my little boy has started to wet the bed most nights, it really upsets him when it happens and i cant think of any reason why this has just started, when he has never wet the bed for over a year prior?

Any suggestions? ?