Is your cat overweight? I have a friend who had this same problem. It ended up being a gland in the rectum. Cats generally "empty" the gland when the are cleaning themselves. Sometimes, however, they cannot empty it on their own. This gland fills up and causes an odor. Take your kitty to the vet and they will be able to show you how to "empty" this gland for your uncomfortable kitty. Hope this helps.