I agree with the others that this is a big red flag. If she is being dishonest here, she could lie to you about other relationships. For example, she could be seeing someone else and hiding it from you because she is not sure how to tell you.
She needs to work on an open, honest relationship or she can create a bad foundation for her relationships.
I would say that you should be very careful with her until you can telll she is being open and honest and communicating to everyone.