My 15 mo. old Buff Orp "Fancy Pants" has been sitting in her nest box for 4 days. If anyone goes near her or touches her she gives a low growl, puffs herself up and sticks her butt in the air.She did not seem to be getting out to eat or drink. She has not been laying so I thought she might be egg bound. Brought her in gave her a warm bath & kept her in the house for 24 hours. Checked her all over, didn't see any signs of mites etc. but did notice that she is missing a lot of her soft downy under feathers on her bottom side. She was eating okay so I put her back with her "sisters." She went right back to the nest box & won't come out unless we physically remove her. She will free range as long as we keep the pop door closed so she can't get back into the coop but once that door is reopened she makes a beeline for the nest box She is fed an organic layer feed, treats of sunflower seeds, oats & yogurt. Her water always has org. ACV & garlic in it. I have five other hens all the same age, no roosters, no one else is exhibiting any strange symptoms. I have no experience with a sick chicken, what could be up with her??