Puppy behavior changes after neuter?

We've had our Bett Hound (naturally low energy dog) since he was 3 months old. Hes always been very active and hyper, typical puppy stuff. Hes now 13 months old and we got him neutered 3 weeks ago, after his wound healed he was back to his usual antics, but now over the past couple days he's totally changed. Before he got so excited he couldnt think straight when you went to feed him, now he could care less and just grazes over his food all day. He sleeps all day and when you get him to go out hes very slow moving. Not as easily excitable as he used to be, yet he still playful when you want to play fetch or something which makes me feel like hes not sick or hurting. This is the Bett Hound I expect, but not still in his puppy years. Do you think he's sick or do you think its caused by his neuter?