Serious move backwards - new term to blame?

[TeaTowelQueen]

My 3.5yr old potty trained himself about 3 months ago after I'd thought he was never going to get round to it, but since he started back at pre-school last week he has practically gone back to square one and I am in despair! At school he stays dry however and also when we are out and about he is quick to ask when he needs to. Anyone else come across this at all? His key-worker left last term and he is attending two whole days a week instead of two half days and I am worried he is not happy (although he is still just as keen to get there each day). Any pearls of wisdom?