First off, hello! The first thing that comes to mind when you describe Kaya's back end swaying and her inability to climb stairs, is the possibility of hip dysplasia. Kaya is an older dog, and she may be becoming symptomatic now for this condition, which is present from the beginning. As for the not eating, she may be holding out for better stuff than dogfood, or she may be painful and not feel like eating much. Hip dysplasia is usually treated either with surgery or pain meds, but with an older dog with lots of arthritic changes to the joint, surgery might not have as good an outcome. Plus it's very expensive to have hips replaced. Without surgery, the treatment usually consists of treatment with a NSAID like Rimadyl. You can also use nutraceuticals like glucosamine, Vit C, etc and some people have excellent results with that.
Another thought that occured is that it could be a neurological thing, that can cause ataxia, incoordinated movements and weakness.
Either way, it's going to be something for your vet to diagnose, since it's awfully hard to do over the internet!
And yes, Pedigree is awful food, and there is a school of thought that says grain based commercial foods are extremely irritating to arthritic conditions. We have two hip dysplastic dogs, one is a GSD with a severe case, and both are pretty much asymptomatic without meds and we feed them a raw diet to avoid grains. The shepard had one hip removed and seems to feel great! I like to avoid meds as much a possible.
Last thought, Baytril is a very strong antibiotic, don't know if it could cause the signs you describe, doesn't seem likely. Baytril is not used much in growing dogs, though, because it can cause damage to the unclosed growth plates. But it's used quite commonly(and way too much) in adult animals.
Hope this is helpful!!