Took Shai to the vet today. Latest symptom is she is passing a lot of blood in her urine. Had this before with Coco and a course of Baytril and Metacam cleared it up.
Millie came along for the ride because Shai gets very stressed if she is parted from her friend. I also took a naice bag of salad leaves, that didn't last long 
We talked over Shai's baldness, weight loss, fluid retention. Ruled out mites and fungal infection. Suggested Cushings Disease. Vet agreed it is a possibility and would also account for water infection. Other possibility is ovarian cyst, but on feeling her tummy the vet couldn't find any lumps.
So what to do. We could arrange for an ultra-sound scan and blood test. The question is then what do we do with the diagnosis? Treatment for Cushings is regular blood tests requiring her to be gassed to sedate everytime. We feel it wouldn't be kind to put her through that.
So we assume it is Cushings and make Shai as comfortable as possibility without treating for the disease. Shai has Baytril and Metacam to hopefully cure the urine infection. She is eating and still happy with life. From now on we are keeping her pain free on Metacam and giving her favourite veg.
I know we only have a few weeks or months with her. Hoping she passes peacefully in her own hutch with Millie by her side. She's been a sweet little guinea.