The baby may nip at the mother's fur, but I suggest taking her to a vet.
My guinea-pig gave birth on new years day and the same happened to her. Someone told me that it may be the baby guinea-pig chewing the mums fur for attention. It is nothing to worry about. My guinea-pig didnt have a cut on her bak though. This may of been where the baby has just bitten to hard. If she carrys on losing hair take her to a vet. Hope this helps.