It could be fleas (it only takes one, especially if they are allergic), food allergies, or stress-related. Is your cat on a vet-prescribed flea medicine, like Advantage, Frontline, Revolution, or Program? I would definitely see the vet for a check-up. They will do an exam and check for fleas. If there are signs of fleas or flea dirt, they can prescribe one of these medicines (Advantage, Frontline, and Revolution are applied to the skin on the back of the neck once per month, Program is given orally in food once per month). If there are no signs of fleas, the vet can help determine if there may be a food allergy or other cause of the hair loss.