A while ago my female fancy rat died. She was lying in the center of the cage and taking really small breaths I could harly see them. The next day she was dead. She had enough food and water though. She didn't freeze to death either. Anyone help me get some possible reasons of her death? :(

It would be impossible to say without pre-death tests (x-rays, blood work, vet exam) or an autopsy. The short breathing indicates heart failure most likely but the heart can fail from any number of problems. Rats often get cancer or organ failure. My guinea pig had a grossly enlarged heart (she had x-rays) and died of a heart attack last month. Unfortunately, as great as they are, rats have really short life spans.

was there any discharge from the nose? short labored breaths could be a symptom of a respiratory infection.

nothing comign out of her nose, nope.
B)