I"m gonna say what everyone told me when I posted about my Eliza girl's spot on her tail..

If it's new, and isn't improving, go to the vet. Could be allergies or lots of things.

In Eliza's case it seems to just have been that she always has a paw there that rubs against the spot when she's bathing her self (she does this because she has short legs and would other wise roll over!) That and her skin was really dry... but her hair is growing back.

Keep an eye on it, and if it doesn't improve, or gets worse, make an appt with the vet. Kitty won't be happy, but its better safe than sorry!