I think it's uncommon for a cat that age to have kidney problems, but you can't rule it out. A vet visit is definitely in order if she's that small so the vet can maybe do a blood panel and check things.
My auntie and uncle had a cat that was absolutely tiny. Noone could figure it out, so she was taken to the vets and it transpired that the poor little mite had a really severe allergy to her own fur and was really ill from it. I've never heard of that before or again, but it does highlight the need to take a cat to the vets if it's stature is that off the mark.
Good luck, let us know how she gets on.