Why does my cat bite when I pet her?

Petting-induced aggression is overstimulation, not personality. Cats have an arousal threshold for petting, and going past it tips the experience from pleasant to overwhelming. Cornell’s guidance on aggression flags this as common, particularly when petting continues longer than the cat tolerates.[1] Watch for the early warning signs: tail twitching, ear position changing (especially flattening or rotating back), skin rippling along the back, body tensing. Stop before the bite. Most cats have a strong belly-rub threshold (much lower) and a longer head-scratch threshold (much higher).

When to call a vet: if a previously tolerant cat suddenly becomes bite-y during petting, that may be pain. Vet visit.