Dear Anonymous,
I'm not a vet, but it could be a cardiomyopathy. I've had a couple of
ferrets with heart issues and they both exhibited what you describe:
extreme lethargy and fluid accumulation (which may explain the
swelling). There are meds to help them with this. Hopefully, the vet
will figure out whats going on very soon!
Best wishes.
Susan
[Posted in FML 6583]