All I can venture to guess is a reproductive issue which admittedly covers a lot of serious stuff. If she's managed to get an egg to rupture inside, she has a chance of clearing it out with the help of the calcium, which assists her in producing contractions to move it out.
If she's got an infection in her oviduct or if it's progressed to the point her system is dumping yolks into her abdominal cavity, her days are numbered even if you could find an antibiotic to treat her.
Then there's always the chance she has tumors in her reproductive tract. I don't need to tell you how ominous that would be. If she continues to decline, that's the conclusion I would finally make.