Chimera has been staying with my parents while I do a long pet sit. This morning my mother called me to report that he had been lying down a lot with his head between his front paws (rather than the more relaxed lying down on his side) and not following her around as much. He'd also stopped walking and hunched his back here and there. I suggested a GI problem and said to monitor him closely. An hour later my dad called - he was now hunching over a lot and his back legs were shaking. My parents both took him to the nearest emergency vet.
It turned out that one of his anal glands had gotten infected, swollen, and burst. It was bleeding when they reached the vet. Fortunately, the vet's assessment was that pain killers and two week of antibiotics should clear everything up.
I wasn't able to drive down and meet them because I had to teach two training classes while this was going on. That may have been for the best, though - I would probably have been a crying wreck in the waiting room.
He's doped up on pain meds but is resting much more comfortably.