Cat pulling out her fur?

Ok, so a female tuxedo cat who lives in Mesa, AZ, indoor/outdoor cat, and lives with 2 other female tuxedo cats, all indoor/outdoor. None of the other cats have any issues, and all are healthy and happy as could be. However, our dominant female, who is part Siamese, is pulling out the fur on her lower back. it is getting progressively worse. whenever she grooms herself, we find small clumps of fur strewn across the house. We treat her regularly for worms, since she is partly outdoors. None of the other cats have flees, so we know that is not what it is, and she has also had progressive weight loss. she is not due for her examination for several months, so I was wondering if anybody knew what is is that is bothering her, and if I can buy it over the counter? she has no obvious ailments besides thin fur on her back end, and inching up her torso. health ears, healthy gums, healthy eyes, healthy eating, pooling, and temper. any suggestions?