[cypher]

I'm so sorry for your loss, iI know how hard it is to lose a hamster so young.
It isn't always easy or even possible to know why they die, especially at such a young age.
I do think diabetes is unlikely, it isn't common in WW hams (it's most common in Russian hybrids, Campbell's & Chinese) it may possibly have been an underlying kidney problem, the weight gain may even have been fluid retention if that were the case.
I can only guess I'm afraid, someone else may have a better idea.
Although it does sound like an underlying problem that may have been congenital it's not necessarily a genetic problem, I would let the breeder know what happened & see if they have any more idea of what it may have been.
You gave her the best care you could, it is very sad but sometimes no matter what we do it's just not possible to prevent things like this from happening.