He could have got a puncture wound which is maybe infected and building up to, say, an abscess. They're easy to get in a fight and surprisingly difficult to see (thick fur, sharp points to his opponents' claws/teeth) unless and until an abscess develops or bursts. Cats claws and mouths are foul so a wound could easily go bad.
Keep a close eye on him - and I'd have him to the vet tomorrow, myself. Part of the trouble could be the shock of a serious scrap. It can take them some hours/days to get over that. (Imagine if you had a full on fight with adrenaline coursing everywhere.) An infection is the likeliest cause though.
Our 6 year old boy came in 2 nights ago, lost collar, smelling quite bad (bit of a pooey smell) and had obv been in quite a bad fight. He fights occasionally with other boy cats in the neighbourhood, I guess its to do with territory. Whenever we see/hear him, we always try and break it up, but didn't hear or see anything this time. He has been neutred, but we know other male cats in the area haven't.
I managed to wash him and check him over, he hadn't pooed himself, which i thought he might have due to the smell, and he had a few bad scratches/small patches of loose fur. He ended up going straight to bed. Yesterday, he seemed fine, eating well, just looking a little forlorn.
But today, something isn't right. He's not himself, not eating, not meowing and looking very sad. He is drinking, but i presented him with some chicken (usually his fave) and he gagged at it! Never seen that before. My plan is to keep an eye on him the rest of the day, and if he's not improved by tomorrow, to try and get an app with the vet.
He's had all of his jabs, but i know this doesn't protect against all the infections out there. Has anyone else has any experience of their cats getting like this after a fight? I'm trying not to worry, but its horrible seeing him not as him usual self.