Why is my puggle chewing on one of her paws?

My puggle is almost 2. For a week now, she has been biting her back left paw. She is literally attacking it, and we don't know why. It now has bite marks between the paw pads because keeps doing this. She is an indoor dog and she is on revolution, and has been her entire life. Her diet consists solely of prescription T/D. She seldom has interaction with other dogs, and we do not have our lawn sprayed with pesticides. We also avoid walking her on lawns where people have pesticide signs. She is bathed on a normal basis, but not too often. She hasn't ever had skin irritations in the past, so we are curious as to what this could be, since it is just the one paw - the other three are left untouched. Any help is appreciated. Thanks much.