At her age it is necessary to have bloodwork done at a minimum and I would suggest x-rays as well. There are various causes for swollen stomach including bloat, constipation, parasites, peritonitis, cushings, pyometra and edema. Bloat usually includes attempts to vomit, restlessness and is pretty deadly if not treated quickly, thus we can rule this out. constipation can cause a swollen belly but you don't mention her not being able to defecate. Parasites are easily diagnosed so I'm sure your vet would have caught this easily with a fecal exam.
In addition, a puncture to the gastrointestinal tract can cause peritonitis which you can read about here:.
Cushing's is when the adreanal glands overproduce glucocorticoid. This can be caused by a tumor, steroids and is mostly seen in older animals. Symptoms include excessive drinking and urination, increased appetite, increased panting, lethargy, hair loss, weakness, and a pot belly. Dog's with cushings sometimes have darkened skin. Treatment and prognoses depends on the cause. In some dogs treatment is very effective. You can read about this here as well as more detailed information on cushings.
Pyometra is a uterine infections and usually occurs 60-70 days after a heat cycle. It is usually seen in older unspayed females but can be seen in any age. It is diagnosed by ultrasound showing fluid in the uterus. There are two types of Pyometra (open and closed cervix) Symptoms of open cervix Pyometra are a vaginal discharge that may be pus or bloody, lethargy, anorexia, increased thirst and urination, vomiting and diarrhea, and abdominal swelling though the dog remains fairly healthy due to the infection draining from the body. Closed cervix Pyometra symptoms are the same without much of a discharge, they appear very ill, there is usually weight loss and abdominal swelling and is much more extreme. Dehydration, shock and death can occur if not treated aggressively.
Edema is fluid buildupand in the abdomen andit could be due to puncture, cancer, heart issues and even liver failure. Along with the other symptoms you are seeing could indicate a number of different things. There are just too many possibilities at this point to pinpoint the cause without bloodwork. I would also get x-rays or a ct scan done as well. I hope this information is helpful to you.