Aww, I'm so sorry to hear this. I've had my last hamster like 20 years ago when I just started school. It died young from a brain tumor and I was absolutely sad. Sadly cancer is a very common fate for all kinds of small animals, especially hamsters, mice and rats. I had rats for the past 10 years and lost a couple to this. You never know, sometimes it just happens. It might have gotten to yours, too. There are a lot of things you can't influence, even with the best care.
Sorry, sorry!
/comfort>Daughter.