As a pet grooomer I have seen cases of badly messed rear ends. When they get to the point that you dscribed, the cat will not be able to clean it herself and it WILL only get worse. If you talk to a vet or groomer and explain that she is a stray that will need to be caught they may allow you to come in with just a simple warning call to them that tells them you have her and are on your way in. (especially if you tell them you ONLY want the rear end shaved...this will help in getting an on the spot appointment time.) Then you can make a full appointment time to have her looked at by a vet to find out if there is a reason it got matted and messed in the first place.