I just got done performing surgery on my cat. He was acting super docile and affectionate the past few days; I figured it was age, and that he was just nearing the end. Turns out that one of his wounds - he gets in a lot of fights - had become badly infected. I went to drain it, thinking it was no worse than any other gash he gets from protecting his turf. This one went deep, though, and smelled god awful once I opened it to drain it.
He's sleeping on the couch now. I'll find out tomorrow if I got all the infection out.