
Chewing at the tail can often be a sign of anal gland discomfort. Also chewing or sucking the feet can be a sign/habit to ease discomfort.
My youngest bitch took to sucking her hindpaw when she had an eye ulcer that obviously was painful. Unfortunately it turns the white/grey fur orange. She does it every now and then when tired or bored (so it has become a habit, even when the original problem was sorted), doesn't chew but just slobbers and discolours it. I resort to using bitterspray on it. Also washing the foot with a clean flannel and water.
At this point I would get her anal glands emptied, and Frontline her in case she has picked up Harvest mites, which can really iritate the feet. I would use the spray and not the spot on, and spray the feet.