Housebreaking regression in 2 year old dog..help!?

i boarded my completely housebroken maltese male for 8 days at my vet's office. they take them out at intervals for bathroom breaks. since we've been back in town (approximately 2 weeks) he has had accidents in the house at least once/day. what in the world would cause this behavior? could a dog regress that much in one week? i was afraid to live him at the doggie day camp because he would be mingling with dogs of all sizes (he's only 5 lbs.) so i left him at my "trusted" vet's office. frustrated!