sorry I didn't see your post earlier....I looked at the pic. your poor poor kitty how she must be suffering with a rash like that.
IMO its a severe allergy to something...since its on her underside it could very well be the litter, or does she go outside?? if so it could be a chemical such as grass fertilizer/weed killer etc....do you keep her indoors? if you haven't please do so.
Since this IMO looks like a contact allergy, what do you use on the floors to clean with? do you have lots of carpets??
as far as kitty litters, what kind is she using? have you tried other brands or something other than a clay?
You can go to a petstore and get paper litter, like a rabbit pellet, you can try corn litter or pine litter too. I strongly urge you to try that first AND BE SURE ITS UNSCENTED!!!
I also have a thread I will include tht I just posted on yesterday and on it I give some links to sites that talk about EOSINOPHILIC PLAQUE/ GRANULOMA COMPLEX also ATOPIC DERMATITIS.
This is becoming very common unfortunately. Its from an allergic response to SOMETHING and it will take some research and trials on your part to find out to what....could also be her food???
heres the link to the other thread, read all you can on there and see if you can make some connections that are similar to her problems, please post back and lets see if we can come up with something to help....
http://www.medhelp.org/posts/Cats/My-2-yr-old-cat-is-overgrooming/show/1359160
also try
www.peteducation.com/article.cfm?c=1+2117&aid=502
♥Opus