Best wishes for Thumper. I wonder what it could be?

I would definitely ASK your vet about that FIP test. As I understand it, it is not a REAL conclusive test. It shows if the cat has been exposed to the virus but that is totally different to the cat actually having symptoms of the disease. MANY cats have been exposed to the virus (especially if they ever lived outdoors) but a much smaller percentage of those cats actually develop the disease.