What's the white/brown goo coming out of my cat's butt?

today i was brushing my cat to get rid of his winter coat and as i brushed above his back feet (not on his butt, below it) and white/brown looking goo shot out and it didn't seem to faze him at all. it smelled horrible and it took forever to wash away the smell. the goo looking stuff looked nothing like worms. my cat has had his shots for worms annually and is about 6 years old and is the healthiest cat you'd ever see. i'm scared for my kitty, what's wrong with him and what should i do? thanks.