DD2 is 4 months and the last few days has taken...

DD2 is 4 months and the last few days has taken to biting me during feeds. What happens is this : happily feeding, then she tilts her head back so the nipple slips between her gums and then CHOMP! This happens a few times every feed and I'm NOT happy. I'm starting to not want to put my boob in that little mouth anymore. DD1 never bit me EVER so I've no experience and don't know what to do. DD2 also has a dummy which DD1 never had so I'm wondering if she chomps down on the dummy and that is why she is doing it to me. Help! What do I do? PS. It all happens very quickly so I have no warning to stop it.