Why did my dog lose the ability to control his bladder after being neutered?

I adopted a 3 month old puppy from a local animal shelter. One of the requirements of the adoption contract was that I have him neutered within a certain time frame. I waited until the week before my deadline to have him neutered. At that time he was nearly housebroken. I could take him outside and say, "Go potty" and he would. After the surgery he began urinating throughout the house but it seemed involuntary. He didn't squat or raise his leg, he did it while walking around. He once left a trail from the living room, down the hallway, and into my child's room. This began happening multiple times a day, every day. I called the vet's office to see if perhaps something had gone wrong but the ditsy receptionist was no help and refused to give a message to the veterinarian. What do you think the problem was? Error during surgery or behavioral issue?