She is going through a teething stage so it is normal . Trust me, my puppy used to do that too but she is now 4 and a half months and has grown out of that habit. When she does nip you direct her mouth to a toy or something else for her to chew on. Teach her (with treats) that leaving your hand alone is a good thing. Ignore the bad behaviour. As I said give her a toy or just walk away. You could try giving a cry like another puppy would but it rarely works.