HOW DO I GET MY DOG TO STOP BITEING ME?

my dog is 4month old (american bulldog-boxer mix). wen i play with her she bites my hands..its hurts realy bad... ive tought her to sit and give me paw/shake hands.... but when she bites me its not aggresive its for play. and when she bites me i yell at her and she starts licking my finger as if shes apolagizing ....... plzz help