Female rat is suddenly acting aggressive

Question
My female rat, who has always been very friendly and came to me when I opened the cage door, is acting very aggressive all of the sudden. When I went to pick her up she bit me and would not let go. She is also being aggressive towards the other female rat in the cage, biting her on the neck and ears; she even bit her own tail when she turned around! She is also staying in one area of the cage, building a nest around herself, and being reclusive, only coming out to eat or pick on the other rat. I know she is not pregnant because she has never been around a male rat and I saw no visible wounds, so I do not think she is in pain. She is not old, either. What is wrong with her?

Answer
How old is she?  This will help alot. ALso how old is her cagemate and how long have they been together?  ANything new lately, as far as new cage, new toy, not taking her out for playtime as much? Its hard to say what set her off but once I know the age of both of them that will help some.