Tux went to the vet today. It turned out that he had infected anal sacs. (eewwww!)
This occurs when the sacs become clogged with the liquid used for scent marking. The vet emptied them and put Tux on a round of antibiotics.

I`m so glad that it wasn`t a more serious condition!

P.S. Thanks to everyone who responded to my thread!

Van10 - Meowmy to a stinky tuxedo kitty