Do you know when she last laid? At 6 years old, with a swollen abdomen, it's likely some kind of reproductive problem and/or organ failure. It can be infection, or cancer, both of which are not uncommon in layers over the age of two. If it feels like fluid, then it's ascites (sometimes called water belly), which is fluid build up in the abdomen from organs that are over worked by disease, like cancer, or failing organs like heart or liver.
The fluid can make them uncomfortable and sometimes makes it hard to breathe, and makes it harder to push droppings out, which is why they will often have poopy vent feathers. It can also affect digestion and crop problems can sometimes be an issue also, from the digestive tract pressure. You can try draining it, but that is not without risks, both of infection or shock if you drain too much too quickly. Draining may make her more comfortable for a time, but it will usually recur since the underlying condition is still there. I generally don't drain, I just leave them with the flock until they are obviously unwell, isolating themselves, go off food and water, or get attacked by flock mates. Then I euthanize.
This is a video on draining the fluid, if you want to try that, there are others if you want to search.
