The two most common causes of hair loss like this are an imbalance of hormones put out by the endocrine system and overgrooming by the cat.
Watch your cat closely to see if she spends a lot of time grooming her tail. If she is, you may have a case of overgrooming. This is often caused by stress. Small things can stress out a cat to the point where they begin this behavior. Have you had a houseguest? Has your work schedule changed? Have you moved the cat's bowls or bed? Think through all that has happened in the past month or so. If there's something different that you can change back to how it was before, that may help. However, some cats begin overgrooming as a result of a temporary stress, such as a houseguest, but the grooming becomes a habit, even a compulsion. In that case, the problem may have to be treated by a vet with antidepressants or other medication.
If the problem is caused by a hormone imbalance of some sort, your vet will need to do tests to pinpoint the problem. Treatment with hormones solves this problem.
While these two causes of hair loss on the tail are most common, there are a number of other possibilities, including parasites, bacterial or fungal infections, and some more unusual diseases. Here is a site where you can read more about hair loss in cats:
The best thing you can do for your cat is make an appointment with your vet to try to get to the bottom of this.
I hope you're able to quickly solve the problem. If you have further concerns, just let me know.
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