Mystery Death of Rat

Question
I brought a male dumbo rat 3 weeks ago, he was about 3 moths old and was perfect. He was very active, good appetite and seemingly healthy, but with in the space of 24 hours he died. We had him out on Saturday evening and he was fine, his usual self. Sunday morning he was quite and seemed sleepy, later that night, when he would normally be out and playing, he was still not himself. We examined him and noticed his testicles were no longer hanging down and he had a bit of a swollen belly, his front end was fine but the back end was being dragged, his reflexes were ok on the back but couldnt seem to push himself along. I put him in his carry cage overnight (with food and water), so he could not do himself any damage by falling, with the plan of taking him to the vets today but this morning he had passed away. I am very shocked by this as it all happened so fast. All my other rats have had long healthly lives. Do you know what may have happened to him as ive never experienced anything like this before and definatly do not want the same thing happening again.

Answer
Its really impossible for me to say what could have taken his life at such an early age, but I am very sorry you lost him so young:(

At his young age, he could have had megacolon, although you said he was healthy up until he died so that most likely rules that out.  With the swollen belly that was so acute, this could mean fluid retention seen in heart disease or kidney failure...
When you say his testicles were no longer hanging down, do you mean he had retracted them?  Males have an open inguinal canal which allows them to draw their testicles up into their scrotum, sometimes fooling rat owners into thinking they have a female rather than a male.  They draw them up when they are scared and when they are ill as well.      

That said, he could have died from various things, probably congenital since he was so young.  I would say it was more or less a bad heart that took his life. The sudden swelling of the belly indicates fluid retention so he may have gone into congestive heart failure.

Its just to hard to really say for sure but alot of times sudden death in young rats is heart related.