Many years ago I read a letter from one cat lover whose young male (I still remember his name, it is Tazz) had the same problem and, after all, it was discovered that he had FIP (Feline Infectious Peritonitis)! It was of so-called dry form so Tazz managed to survive! But, if your cat is strictly indoor, it is hard to believe that she acquired that virus. Maybe it's worth checking your cat for the presence of retro-viruses? I really hope her disease is not so horrible.
Many regards from Deda Brada.