Thanks for all the replies. She's had it for a while, but not since hatch. I first noticed it a couple of weeks ago, but it seems to have gotten worse. We use a plastic feeder, but there is chicken wire everywhere, so it's possible she's scraping against something. It's also possible she's getting pecked, although I haven't seen it recently.
Sorry to hear about your OEGB, NottinghamChicks. I hope that's not what it is. I guess I'll try some topical antibiotic and see how it goes. She seems to be behaving normally as of right now.
The only thing I can think of is that a couple of months ago, she was taken by a predator. Somehow, she got away and returned five days later, but it's possible something happened then.
If anyone else has any ideas, please let me know.