My cat loves a stroke and he lifts his bum in the air when I stroke him normally.
Just give him a good back scratch now, and noticed he is straining his bum slightly and there’s something yellow in there, like a small ball or button (so not stringy like a worm)
Not that I stare at his bum a lot, but I do check from time to time to make sure he’s not sore if he’s been overgrooming for example, and I’ve never seen this at all.
Google tells me it might be an anal gland, is it normal to see it when he’s lifting his bum up? You can’t see it normally, just when he ‘pushes’ his back up if that makes sense.
He seems fine generally.