Sometimes it can just be anxiety. It could also be a thyroid problem or Diabetes. In any case, you should see a doctor and have it checked out.
Things like this can happen to anyone, no matter how healthy.
Slaughter.
Asphyxiation
There are several causes of sudden water retention. Poor circulation, premenstrual symptoms, salt, or a blockage in the lymphatic system can all cause this condition.
a surge of testosterone in males and it causes ovulation in females.
testosterone
Ingesting Ipecac
Improper diet
A shock is something which causes sudden, heavy impact, a sudden electrical charge, or an amount of considerable emotional impact.
I would ask your vet but that sounds like Parvo
oomycete plant pathogen
The sudden supersonic expansion of material.