One of my ferrets, Eve, has the exact same thing: a large, distended belly
with no discernible lumps and no pain.  We took her to the vet, and x-rays
showed no large masses, lots of fat around her kidneys, a slightly enlarged
heart, but everything was relatively normal.  The vet did an aspiration to
see if there was fluid, and there was--a yellowish fluid (not urine).  On
the slide, though, there was only the occasional large cell, so she sent
the sample in for a more thorough cytology.
 
The cytology came back negative for cancer, some signs of white blood
cells, but generally inconclusive.  Eve is active, curious, and chipper.
The Vet recommended that our next step be an ultrasound, which we are going
to wait on for a while to see if her condition changes any over time.
Without further tests, which if nothing shows on the ultrasound, would
involve opening Eve up to see what it is (which we're not too thrilled
about), her condition is pretty much undiagnosed.  So, I can't really
help, but I can empathise.
 
-cobalt
[Posted in FML issue 3475]