OMG..... get her to the vet NOW!!!!!!
I had a dog, gorgeous great dane only a year and a half old. Got the exact same symptoms. It was a holiday weekend and after awhile he seemed to settle down. Well he was dead by morning.
Next dog was a lab shepard mix. He got the same distended belly symptom and this time I took off work and rushed him to the vet. He was there for three days. It was touch and go for awhile but he made it.
This can be VERY serious. If a dogs belly swells, and it could be from anything including something he ate, (mine got to and snarfed down a half loaf of bread.) it can twist and that is instant death to a dog.
I hope it's not it, but this is nothing to fool around with. Only a vet can determine exactly what's going on.
I am sure these symptoms could be the aging process. However, she could have congestive heart failure and is retaining fluids. She could have liver or kidney issues going on. Some of these may be treatable. You really need to get her to the Vet for an exam and have blood work done to try to determine the cause and if you opt to choose any treatment.