My 11 month old DD recently started biting my nipples when she's mad at me. I always nurse her down for naps and at bedtime, and when she doesn't want to go to sleep, she bites the nipple. Now that her upper teeth have come in, it really hurts. I've just been saying "ouch, that hurts mama," and then putting the boob away. But that doesn't seem to be working. Any suggestions?