Does your bun run on carpeting alot? Is there one sore or multiple? About how big are they? These arewhat arecalled "sore hocks", most likely. Are they on the heal of the bun's foot? That's usually where they appear, but not always. If it's not too raw, you can put a triple antibiotic ointment on it. You can also wrap it if you feel it's necessary. You will want to keep your bun off urine, that will only make it worse. You can use one of those canvas screens to cut to size and place in the litter box.
My mini rex had one when I got her, but was cleared up, the skin/callous was just a bit exposed and it was very light pink, pale actually. When they get irritated, they get more pink looking and the fur disappears from that area, but surrounds it.
