A little nip is a cat’s normal way of saying she loves you, and a common sign of affection. This is their natural instinct, and they learn this from a young age, since kittens nip each other affectionately.
Your cat might be giving you a good cleaning. Cats do occasionally bite during the grooming process, which usually happens when they can’t quite get something off their fur or they need to scratch an itch.
Maybe you were petting your cat, and this was pleasant, but then turned to be uncomfortable for her. Since cats can’t use words – they will show you their dissatisfaction with a little bite!