I suggest that you spend a little time with the chickens over the weekend to see if she is being picked on. If she is the thinnest one she may not be allowed to eat if the others are bullying her. I have a girl who gets bullied away from the feed bowl, so I put in another feed and water bowl specifically for her, and try to make sure she gets some treats thrown her way. Of course, others use "her" bowl too, but it does help.
You didn't say how old your chickens are. If they are at least a year, they could be going into a full-out molt. If younger than that, could still be molting but not as extreme. Good luck. I hope it's just a molt.