Bald patches under tail, by anus?
The only time he's EVER dragged his butt across the floor (that I've seen) is after he used the litter box, because he had poop stuck to his butt/fur. I'm pretty sure he doesn't have fleas and he's a 100% indoor cat. He's overweight (I've been trying to diet him, as the vet instructed) and about 7 and a half years old. My building is old and has these insects that I think are house centipedes, it's possible he could've eaten one.
I'm planning on calling the vet tomorrow, but I was wondering if anyone had any idea what this could be?