3.5 year old potty trained boy weeing ALL the time!

[fernie3]

he doesnt seem sore or to have any symptoms of an infection of any kind but the last week he has gone from being totally potty trained day and night to having around 4-5 accidents a day. He goes to nursery and has come home with two pairs of wet trouers in his bag. He can just be sitting there and weeing and not even notice!
Any ideas whats going on here?
I dont want to make a huge fuss out of it in case it makes the whole thing worse.

My 5 year does this sometimes?! Even though she's been potty trained since 2 and normally fine night and day. Sometimes it IS triggered by a urine infection, and it's stopped when she's had antibiotics. Other times we can't get to the bottom of why, and it just stops after a few days. She's had bladder scans and tests..all normal, no emotional issues, just one of those odd childhood things. Some days I think she's just too involved with something and leaves it until the very last minute..by which time she can't hold on..and once this has happened we tend to have a run of it for a few days. The only things that helps then are regular reminders, or just actually taking her every couple of hours. I don't think it's entirely unheard of for this to happen, but perhaps check with GP in case underlying infection?
Does it just happen at nursery? Is there a practical problem about him asking to use toilet, or just too busy playing to be bothered to go in time?
Hope you sort something out..good job there are washing machines in the meantime

It may simply be that he is so engrossed in play that he is resisting going and doesn't make it.