Hello and thank you for asking your question on just answer. My name isXXXXX will try to help.
Did her stomach suddenly swell today or has that been going on for a while?
What color are her gums?
How is her breathing?
How much does Molly weigh?
It does not sounds like "bloat" or a twisted stomach. When they have that, they vomit repeatedly until they start dry heaving. Then they keep dry heaving and can not stop.
There are a lot of things that can cause her abdomen to distend.
Most of the causes are fairly serious. She could have right sided congestive heart failure.
She could have liver or kidney disease. She could have tumors in her abdomen or chest.
There is no way you can diagnose this at home. She needs chest x-rays, and abdominal x-rays, to check for heart failure and tumors. She also needs blood work to check her liver, kidney, spleen, and for internal bleeding.
If she has heart failure, there are medications to help her heart work more effectively.
If she had tumors, they may be able to remove them but if they are large then the prognosis is poor.
If she has liver or kidney disease, it will have to be treated symptomatically but usually can not be cured.
The arthritis would not cause her abdomen to distend.
So, I am more concerned with her abdominal distension than I am with her arthritis.
She needs to see a vet first thing in the morning if not today at the emergency clinic.
Urinating in her sleep could be because she is incontinent and the muscles can not hold the urine in or she is drinking so much water that she can not help it. Could be kidney disease, adrenal gland disease, or diabetes.
Does her swollen abdomen come and go as well?
See if you can get her to eat some human food. Try lunch meat, peanut butter, or cheese. You can also try canned dog food or a little cat food. They love cat food because it is all protein.
You can also give her a tablet of pepcid ac, or zantac, to reduce the amount of acid in her stomach which should help settle her stomach.
I think the vet will be able to run tests and give you peace of mind.