Rainy, my RIR hen had the same problem. I trimmed the poopy feathers around her vent. Her skin looked great, she was eating, but one thing I noticed after she was cleaned up, she had squirty poop. Diarrhea, but not watery, just really loose poo. It was not clearing her feathers when she would eliminate and it would get stuck. I decided to worm the girls because we had wild turkeys visiting our yard. I wormed them with SafeGuard 1 week ago. Three days later her poop was getting better, after a week, it was MUCH better. I wormed them for a second time yesterday. I never saw worms. She has a healthy weight and was eating/drinking normally. Worming did the trick, though. Now her butt stays clean and her poop is formed.
Just a thought. I hope your girl is better just with a little feather trim.