Mom of baby chinchilla biting its base of the tail !!!

Hi, so I posted a thread of what to feed my baby chinchilla a couple days ago. The mom has been producing milk but I’m worried about the baby because the mom has been biting it’s base of the tail. I’ve seen her bite it!! It doesn’t even have hair there anymore!!! What can I do to make her stop that? Or what I can do? I have separated the baby from the mom and just take it when he needs to eat. I’ve been trying to feed it just in case he doesn’t eat enough and so far seems good and active. Thanks in advance :).