Usually when cats lose their fur from their back, careful examination will reveal some broken hair shafts. If they are present, it means that the hair is not dropping out, but is being taken out. This is almost always simply that the cat is licking those areas and her rough tongue is gradually breaking the hairs and removing them. Therefore treatment revolves around reduction of licking. You would need to examine your cat carefully for this, and if in any doubt, your vet is best able to decide.
Licking excessively like this is usually done when all is quiet, so owners never see it and might not believe it happens. The licking can be caused partly through being itchy (fleas being the main cause) or partly as a "silly habit" which becomes a self-comfort technique for cats which are feeling stressed or anxious. The most stressful thing for a cat is usually other cats, though it can be difficult to help with that. Stress treatments for cats involve environmental changes, along with products like Feliway and Zylkene.
You should think along the lines above, but nothing is as good as a check-up from your own vet.