I lost a Boxer pup at 4 months of age due to heart disease. In her case the abdomen had swollen but if your pups abdomen had gone down that wouldn`t have been the case for in heart disease that is a sign of heart failure. In my breed it is a hereditary condition but as mentioned any purebred or mix can have heart trouble. If you further want to have it checked, the vet office can do a chest x-ray, ekg. and an ultrasound to know for sure if the heart is pumping as it should.