topcoat falling out in spots

Bella has a black stripe on her back, and under it is a red/orange undercoat. Lately, she's had spots in her topcoat fall out, so I can see the orange. She's eating Felidae food, and she doesn't have mites or fleas. What could cause her topcoat to fall out in spots? There are just three spots, each about an inch from the other, in the middle to back of her back.

Could she be over cleaning herself??? DO you have pictures?? It is likely an allergy but a vet should check her out