Do you know what may be causing the diarrhea? Did she let you groom her before she got it? She may be feeling unwell, and wanting to be left alone. If you have not yet done so, a vet visit may help pinpoint what is wrong.
I have taken her to the vet and she was put on steroids. She just seems to get diarrhea occasionally. She is an adopted cat and was abused, so that it most likely the problem. She occasionally lets me brush her on the back only, for short periods of time.
I'm kind of having the same problem. I recently adopted Princess (female, 8, long hair) and she had diarrhea for the last 4 days. Her butt really smelled like poop so I gave her a shower. Lo and behold, she had poop stuck to her (not sure where....). I managed to put some cat shampoo on her and clean her back, but i also somehow managed to scrub her butt only a few times.....I'm not even sure if i could get all the shampoo out....she also won't let me dry her.
Any who....i'm just interested in knowing how to deal with cats that don't like to get groomed or bathed too!