It is certainly less fun when a handicapped pet has diarrhea, there have been some memorable stories here, resulting in baths and housecleaning. I think in this case I might treat it the same as you would when a non-handicapped cat gets diarrhea. Take her for a check-up, and the vet may prescribe Flagyl, if appropriate. Cats don't like the taste of it but it works really well and quickly. It is possible (maybe even likely) it is just a kitty thing, not a handicapped thing. She might have eaten something when hunting, or drank out of the dog bowl, or who knows what. I had a kitty get epic diarrhea after eating a moth one time.
If you can afford it, I would take her to the vet, see if they will do a fecal (they put a swab in her rear and check for microorganisms), and find out if it's just some kind of a tummy bug.
Something else it might be, I suppose, is some kind of common germs in the environment, like giardia. There is pancreatitis, but you'd probably notice a change of appetite. There are various cat viruses more common to unvaccinated outdoor cats, but with those I think you might notice other symptoms as well, such as runny eyes or not feeling well. The only thing I can think of that would be incontinence-related is if she has developed megacolon, and has a partial blockage, and the only waste that is passing around it is diarrhea. The vet could palpate her abdomen and see if he/she can feel any quanitity of hard stool.
Can you take her to the vet? My hope would be it's a common thing easily fixed? That is cool about everything she is able to do and enjoy a full life.