My 9 month old cries violently when I say "No!" in a louder-than-usual and stern voice. This happens when she bites me while nursing. Is it too early to discipline her?

I agree that it is not too early to teach your baby not to bite you while nursing. Like what the others have suggested, it is best to unlatch and communicate to the baby (in a calm but firm voice) that she is hurting you by biting and that it is not appropriate. Your baby would be able to understand from your tone and soon learn what you are trying to teach her. Perhaps your baby cries because she was startled by the "louder-than-usual" volume. Try telling her in your normal voice but maintain a firm and stern manner.