I sort of hate posting this, because I don't want to alarm you since there could be nothing wrong but on the other hand you should be aware of two serious conditions where a symptom might be a swollen belly.
The first condition one of my yorkies had it is also a condition that seems to hit the yorkie breeder more often than most other breeds. My dog seemed 100% normal until one evening when I was giving him a bath, it's when I noticed that his stomach was very bloated, I took him to the ER vet and they drained about 4 oz of fluid. The vet felt it was lymphangiectasia, turned out he was right, so my Duffy was treated with steroid medicine every day at first and then once or twice a week. The other condition is called bloat you can read up on both conditions and see if you feel your put has any of the symptoms. But if his stomach seems to be the same or has gotten larger you should definitely go to the vet
Lymph Node Inflammation, Intestinal Tract (Lymphangieasia) in Dogs | petMD Dog Bloat: Symptoms, Causes, Treatments