Why is my cat over-grooming one spot?

Three buckets to consider. First, pain in that area: cats often over-groom where something underneath hurts (joint, urinary, abdominal). The grooming becomes the symptom you see, not the underlying issue. Second, allergic itch: flea allergy dermatitis, food allergy or environmental allergy can produce localized itching that the cat tries to manage with grooming. Third, stress or anxiety: psychogenic alopecia, where over-grooming is the coping behavior, often in indoor cats with environmental triggers (new pet, schedule change, multi-cat tension). Each needs a different fix and the diagnosis is usually a vet workup with bloodwork plus an inspection of the area.

When to call a vet: this isn’t something to wait out at home. Vet visit early; over-grooming patterns get harder to break the longer they’ve been running.