It is possible that she is pregnant if she was kept with male rats prior to you bringing her home. Especially, if she came out of a pet shop. It is said that rats ARE sexually mature at a minimum of 5 weeks, I personally, I have seen rats that are no older than 6 weeks pregnant and give birth. It is really not good for them, and usually they dont make good mothers that young.

That being said, how is she eating, does she eat alot?

Is her poop runny or not very solid at all?