Why does my cat nibble me when I scratch her back?

That depends on where you're scratching. If it's not too far up the neck, it's a happy kit asking to play. If it's near the tail, tho, they think of it as a painful spot and tell you to stop. Alternatively, the kitty does have back problems and should be taken to the vet. Or... maybe they're just tired? Or doesn't like having exactly his back scratched, but prefers the ears, neck or tummy?

Sorry for this many variants. Not a vet. My older girl does not like being scratched near the tail, so we may be in the same boat))