Number one advice, as an experienced pet owner with 20 years g'pig keeping experience,do not believe what pets at home tell you about her age.I have a lot of experience with their bad information, but luckily have the experience to know when they are wrong.
She is more than likely older and will be more dominant . You may have to take this really slowly by introducing for short peroids of about 15 minutes supervised.
Was she taken from a group of G'pigs at the store or by herself?? If she was by herself this was probably a re occuring problem.
It is a touch and go situation. It is more of a problem with unrelated and related mature males than females. another thing you can do is rub some of her soiled bedding into the none dominant females coat so they smell alike and feed them fresh food at the same time to get them use to eating together.