Recently neutered cat getting infection?

My 6 month old kitten just got fixed yesterday. He seemed to be okay but today he's acting a little strange. He goes back and forth from being really lazy/out of it to being super hyper/aggressive. He's chasing imaginary bugs like he's hallucinating. But that could be just because he's a kitten. He keeps licking his incision site and it's really red. But I don't know if it's swollen or if that's just normal after a surgery. I did remove all his litter and replaced it with newspaper so he wouldn't have gotten anything that way.