Your cat is suffering from flea allergy that is why she is losing her hair. To treat this kind of allergy you can give it antibiotics. Use of special kind of shampoos can cure it from this disease. Some cats also suffer from hair loss due to “Follicular Dysplsia”. That can be caused by improper nutrition so you give it proper food. Wash cat coat with some good antibiotic soap to get rid of this allergy. Also, give it some food supplement that can fulfill amount of nutrition required in her body.