If your cat is experiencing fear or anxiety, while being stroked, it could result in hissing, spitting, or high-pitched cries. Their body will also show signs of what they’re feeling; a fearful cat will have an arched back and its tail up and puffed.
Confident kitties will usually ask to be petted when they need it and an arched back is a typical response. An insecure kitty might have a hard time trusting their owner and will arch their back in confusion, unsure if they want to be petted or not.
While there are plenty of cats that love back scratches, there are kitties that show their bum as a sign of pain. If your kitty trusts you, she'll want to show you what they like and dislike, so an arched back could be their effort to tell you to stop.