I had a similar experience with my Shih Tzu, Jasper. Nothing like that at all when I had Rowdy neutered. I wondered if 1) it was because Jasper was older when I had him neutered (he was over a year old where Rowdy was only 10 weeks) or if 2) it was because Jasper, too, was really active - wouldn't lay still, kept jumping, etc. The swelling did go down and now there's no difference between the two but it sure was worrisome.