I'm so so sorry this happened to youits always sad losing a pet...
As the others have mentioned, some more info about how she was being kept would help us help you figure out what may have happened. When was she last fed? Do you feed frozen thawed or live? If she is fed frozen thawed, how do you thaw the rat/mouse? What kind of substrate was she kept on? What are the tank temperatures? How the heat is regulated and what appliance you use for heating (UTH, CHE, heat bulb, etc)? If she had a bowel movement in her time with you, when was it and did it look normal?
I know we're pounding you with questions but it would help us gain insight into what did or didn't kill your animal. Of course, it's all speculation without a necropsy at the vet.