Hi. . .
Is she chasing her tail and biting or is she chewing on it when she is still as in an irritation of some sort?? If you put down a little more info you may find some sound advice on here. . if you are unsure I'd pop to the vets to see if its a medical or behavioral problem

. . once you know why shes doing it you should then be able to work on the cure

Hope this helps