Poop trails have always been a precursor to something medical in my flock; is this regular for her, or a one-off? If you suspect anything 'reproductive side' might be up, like eggbinding etc, I would recommend a warm soak with epsom salts.
In general for bathing chickens, you can use no-tears child shampoo or any cat\dog shampoo. Blow dry on low, just be aware this can take a long time.
Parasites was a good suggestion as well; when was the last time you had your flock tested for worms? Fecal floats are pretty routine and can be done at any vet clinic.