Maybe another option:
Do your birds free-range and could possibly have been visited by a hawk? Any lost feathers in the yard?
In that case, she may have wounds underneath her feathers. Particulary punctures and tears. But you wouldn't necessarily see them.
I had a white silkie attacked by a Coopers Hawk a few weeks ago and you couldn't see anything from just looking at her. Well, apart from some caked blood on her face.
I put her under the shower and got her completely soaked. That was the only way to find out that she had multiple puncture wounds, a long deep cut on her back and under her wings and whole chunks of flesh missing from her butt. She also seemed to be blind and had one eyeball intact but filled with blood.
I cleaned all wounds out with a lot of water under the shower, desinfected, gave her antibiotics and kept her inside for a few weeks.
She regained her eyesight within 4 days and now after 3 weeks, she's back in the coop with her friends and started laying again yesterday. You can't even see any scars under the feathers anymore!
So, if your girl seems to enjoy the bath, maybe it's worth getting her wet all over and giving her body a good full inspection.
Good luck