No, not a 'normal' diabetes related thing...I tend to agree with vet about congestive heart failure. My civie Scruffy (GA) had the same thing - vet put him on enalapril and lasix. Fluid normally gathers either in their lungs or their abdomen, Scruff's lungs stayed clear until almost the end over a year later. When his was bad, the fluid even gathered in his legs and chest under the skin. Even if that's what it is, it CAN be managed fairly easily. My GA ShihTzu also had CHF - she too took the same meds.
BIG HUGE HUG,