Vicki,
I'm not sure whats actually causing it, but I know how to prevent it and how to fix the worn spot. To fix the worn spot, you'll need to buy some mole skin and stick it in your shoe, over the worn material. This should keep the material from giving you a blister now that its worn. (see my note below for an alternative fix)
To keep the heal from sliding up and down excessively in this area which it kind of sounds like is happening and caused the wearing in the first place (although I could be wrong) you should try the runner's knot. This will secure your heal by pulling more of the shoe's material together without making any other area of the shoe too tight. I hope this helps-let us know.
*Note, I personally wouldn't use mole skin. I would use duct tape but thats just me. I'm one of those guys that fixes everything with duct tape. The reason in your case is the outside of duct tape is smooth and would be a good friction barrier inside your shoe to make sure your heal slides up and down against it.