My hamster recently died and I don't know why.?

I've had my hamster, Skittles, since late last year, and she died last night. I don't know why she died as just yesterday she was perfectly fine. I always make sure she has fresh food and water and that her cage is clean. She got plenty of exercise and I always checked on her when I get up and before I go to bed. Her fur was clean, her eyes were bright, and she was a little chubby. She was a Russian hamster. Any help as to understanding why she died so suddenly will be highly appreciated.