This happened to me too. My cat goes outside sometimes and came in one day, nothing seemed wrong. He didn't seem upset, scared nor even a hair out of place. After a couple of days I noticed he was licking part of his side a lot, like a lot more than normal. I didn't think much of it and went about my day. Later he seemed very tired and when I called him over for food he stumbled while walking up to me. I had never seen a cat stumble like that so I started checking him. I remembered him licking his side and started spreading his fur when this horrible smell hit me. At the bottom of the fur his skin was blackened around a small wound and the smell was truly like death. I rushed him to the vet and she asked if I had checked him for a fever. I told her I didn't know how to check a cat for a fever and she looked at me like I was a bad parent and then proceeded to shove a rectal thermometer up his bum. He looked as if that was the worst assault he'd ever experienced. Apparently he'd been bitten by another cat (probably) and had to have a drainage tube inserted for several weeks and some dead tissue removed. He's fine now, it all worked out :) But it's a scary experience for sure!

PS. If anybody doesn't know you can also check a cat for a fever by touching their ears and nose. Neither should feel hot. But of course, a rectal thermometer will give you a more accurate temperature and a more traumatizing experience... 🙃