I too hope it's nothing serious. If no other symptoms, Chestnut's changed behaviour might be just "old age"! Kitties age on a different scale than we do, that is, much faster, but I guess you know that. It is very common that the muscle disappear a little, and a little arthritis is also very common. Good luck on the check-up! I'll include Chestnut in my evening prayer.
Love,
Sus