sorry your little one is feeling like this.  Hot Rod had some bruising going on when he was neutered.  If the site looks red and angry (like there is oozing) then I would get him back to the vet.  Your picture didn't look like that though.   
Is your little wimpering or anything like that?  If so, call the vet back and ask again about the pain meds.  I agree, I would think the antibiotic is what is giving  him his upset tummy.  Is he supposed to be taking it with food?  I can't remember but Hot Rod didn't like his and he was supposed to take it with food.  It was hard to get him to eat as he wasn't feeling real well.   
Good luck, hope your little guy starts to feel better soon.        |