sick chicken?

you should keep her isolated from the other birds - pen off a small area in your garden for her. Ensure she has plenty of fresh water and give her some layers mash as it is easily eaten. Put some apple cider vinegar in her water (it is a pro-biotic and will help her gut) and give her some brown bread with marmite on it (full of B vitamins) She may have worms - often causes smelly poos - get her some worming stuff from your pet shop and give that. Check her pen and run for mites - they drain the energy from the birds, maybe she had mites and was run down, so more susceptible to another illness. Failing all that, buy some anti-bioics, you can get these from vet or online - make sure its a broad spectrum chicken anti-biotic that you are allowed to give to birds (giving non-prescribed vet meds is actually illiegal in the UK)

hope this helps - you can message me at chickens@little-miss-knowitall.co.uk if you want any more advise.