If the vet has seen her recently then I would give him/her a ring, or the veterinary nurse, and see what they have to say. It could be fleas, alopecia, stress, over-grooming, an allergic reaction to something (any changes recently in food, washing powder etc?).
Have a look at this link and see if any symptoms ring a bell. I do hope you get it sorted. May well be a process of elimination. Good Luck and hope she's OK.
http://pets.webmd.com/cats/diseases-hair-loss-cats