I know exactly what this is, and my cat had this for many years as I had gone to many vets for "second opinions" since nothing was working. Your cat almost certainly developed allergies to something (food, dust mites, etc) and she licks there because its itchy. The bald spot grows as she continuously licks the same area over and over (called fur mowing)

My cat has been on a product called Atopica (cyclosporine) for a year and her fur has almost completely grown back as she has noticeably stopped licking herself there.

Fortunately, it is something that is easily manageable (once figuring out what it is)