How did the hamster die?

Before she died she: *Ate a small cucumber slice *Had small, light brown mushy stool (larger than usual) *Was dropped *Bit more than usual *Wasn't cleaning herself as often (she was active and normal literally a day before she died) *I found out she had food hidden in her playpen which she isn't allowed to be in often so maybe she hid all her food there and couldn't get anymore? *Was climbing bars (she had an above 450 square inch cage, never done that before and was a drawf) *Active way more in the day and night We only had her 3 months, and found her dead in her sleep :( I know she didn't die from old age because she was from a petshop and most animals there are sold young. Yes, my care for the hamster was correct and up to date. Ok so the hamster was dropped by my sister who fed her oxbow while I was on vacation. She told me she dropped her because she bit, and dropped her probably 1/2 feet onto the concrete. (yes I have concrete in one little area of my house) One thing I'm thinking is maybe she broke a tooth and couldn't eat? Oh, and my mom thinks she died from the cold because I did keep the fan on in my room. If thats the case, than I blame myself and feel terrible...