UPDATE:
Thank you everyone for your kind thoughts and words in the comments, our pets really have a way of touching our hearts, don't they?
Our little cat Mooshy has died. She was a beautiful and gentle cat. She had lunge cancer, which I didn't realise could happen to cats. She was very ill and had to be euthanized.
In December she started losing weight very rapidly but had her normal appetite. We took her to two different vets and run tests, but both times nothing was detected. Both put her weight loss down to her getting old. Then, this last week, she had grown thinner and stopped eating altogether and looked so unhappy, so we took her back to see the vet, expecting him to say the same thing, that she was just getting old. Unexpectedly, after another morning of tests, he told us how very unwell she was. She was only clinging to life and in a lot of pain. We didn't know she wouldnt be coming home with us when we took her in, so it was a suprise. It's always sad to lose a much loved pet, especially when they have been in the family for 15 years.