First take a deep breath and calm down. Your kitten is not unusual. Lately there has been thread after thread about kittens doing just this very thing. The bitten off piece of the nipple will probably come out the other end so prepare to become a poop inspector. IF the kitten shows signs of stress, doesn't poop, cries - that's a different matter and a visit to the vet is needed.
Other members will be along to provide more information.