Hi,
I had a patient with this problem and I believe it turned out to be inflammatory bowel disease. We kept it under control with a periodic injection of steroids. It was kind of interesting because she could go several months after an injection without any problems. But as soon as the owners noticed it again, they'd bring her back in. Then I moved out of that state and I don't know what happened to her.

The clear fluid is mucus and indicates some sort of inflammation in the colon/rectum. A coccidial infection (a one-celled protozoal organism) could also cause this inflammation and it would be worth having your vet examine a stool sample along with the clear fluid.

If you have changed diets lately, that could account for inflammation and I would switch back to the other food.

So I would have your vet examine her. Even though it doesn't sound like an anal gland problem, I would think your vet would express those and make sure they weren't causing any problems. At the same time a rectal exam could be done to feel for any growths. Take in a fresh stool sample and some of the clear discharge if possible, especially if it doesn't always happen. I would also expect your vet will do some bloodwork to check thyroid, liver, kidneys, etc in an older cat. But I bet that will come back normal.

Good luck!