I know EXACTLY! My Y.Lee did that, and he was not even a nervous cat. It is anal gland fluid. The best way to prevent the glands from emptying where you don't want them to, is to have them expressed on a regular basis. Your vet, or a pet goomer can do this for you. You could learn to do it yourself, but be prepared for a stinky mess.

By the way.......WELCOME to pet talk, you will love it here.