I tend to agree with Wyorp, not to keep switching antibiotics. At a guess, the antibiotics (combined with reduced food) could be making her droppings go like that. Meloxicam should not be given for more than 5-7 days.
Given her age, and symptoms, it probably is heart failure and lung issues, and really, antibiotics will not cure it. Nothing will cure her, just make her has comfortable as possible, cuddle her, talk in quiet soothing tones telling her you love her so much, and what a great chicken she has been all these years.
And as hard as it is, I suspect this is her nearing the end. I work on a 1:10 chicken to human years ratio, and she is the equivalent of a 90yo human. I have one of similar age, just over 9 years old. Every time she looks a bit down, I tell her she is the "best chicken" and she perks up again. There will come a time when she will return to the "big coop in the sky", and it is inevitable. Whether they are chickens, budgies, cats, dogs or other critters, it is always a hard loss.
Her time seems to be drawing near. Just be with her. Keep her comfortable, comfort her with your words. She will live on in your heart. Be glad you had so many years together. She is lucky to have had such a caring human in her life, and it will be hard for her to leave you too. Just be there for her at the end.
My heart goes out to you.