We had the same problem with our 3.5 yo... only it went on for a few weeks! There hadn't been any changesin routine, we had finally figured out not to let him drink for a certain period before bedtime, and we tried everything- including making him help with taking the sheets off & washing them so he could understand the nasty mess! Definitely make sure you have a mattress protector.
Finally we figured out that he'd been getting up & getting drinks in the middle of the night, so we locked the water dispenser & put the milk up where he couldn't reach it. We also started a potty chart with one of those big desk calendars. We hung it on the wall in the kitchen & each morning he gets up dry, he gets to put a sticker on that date. It teaches him about reading a calendar, the passage of time, and gives him a reward system that he gets so excited about. He actually tells me when he comes to find me in the morning (my hubby wakes him up) that he got up dry that day & is so proud of himself! =O)
Hope this helps... Best of luck!