Did she take a large bowel movement? What kind of rats are you feeding(size & live/pk/ft)? Also do you have another male who may be large enough to breed? If so, take his sperm plugs and wipe them down her back when you try to breed and also maybe try and knock the humidity up in the tubs when trying to breed them by either misting or just tipping the water bowls.
Yea, probably due to a large bowel movement or lots of very large urates. Definitely agree about the not eating thing though.