There is definitely something wrong with the chicken, but she has very generic symptoms that are impossible to diagnose without an expensive veterinary work up (xrays, blood work, stool samples, etc.). If you want to treat at home I would do the following:
Keep her isolated and warm
Take a poop sample to the vet if you can, it is not expensive and it's worth finding out if she has internal parasites - if she does you should worm her and the rest of your flock.
Check regularly for external parasites - mites and lice, because sick birds get them more easily
Her symptoms may be due to pain. To find out, I would try to give her a pain reliever. You can search BYC for information on giving aspirin, I have not done it but have read here that it can be done. If you can get your hands on prescription pain relievers, much better than aspirin are: Carprofen ("rimadyl") give 1/4 of a 25 mg tablet once or twice per day, or (this is an opiate and much stronger) Butorphanol give one 5 mg tablet up to 2 times per day. If she seems better after giving a pain reliever then you know she is in pain for some reason. I have seen this in my chickens who have had arthritis, internal laying, and cancer. One of my girls is disabled from an injury as a chick, she has chronic pain and she gets both a 1/4 tablet of carprofen AND a 5mg tablet of butorphanol every morning, it has extended her life for a few years so far.
Metacam is another pain reliever that is good for birds, it is like carprofen but not quite as effective in chickens.
To give my chicken a pill, i cut a grape into quarters and slide the pill into a 1/4 piece of grape. She gobbles it up. We've also used tiny pieces of cheese and bread. Works great. For aspirin I have read of people mixing it in with their drinking water.
If none of the above give a clue as to what is wrong with her, you could consider an antibiotic but I'm not sure which one to suggest - you could get suggestions here. This is controversial because we don't want to give antibiotics when there is no bacterial infection, which is hard to determine without more specific symptoms or a trip to the vet.
Of course many would cull a chicken at the first sign of something wrong to protect the rest of their flock. My chickens are pets so I take them to the vet when they are sick, which is how I learned what the common treatments are for sick chickens (I'm also a former vet tech). I've paid lots of $$ for veterinary work ups and the treatment in the end is almost always the same - keep warm and isolated, check for parasites, give pain killers, antibiotics, and injections of fluids under the skin if they are dehydrated (I now do this at home too). Good luck, I hope your girl gets better.
