Possible cause of death

Question
I had two rats, one hooded and one sandy coloured dumbo.  They were apparently from the same litter and both seemed really healthy.  Yesterday the dumbo rat passed away suddenly, she was 18 months old.  
I let them out of their cage to run around and feed, they both came out of the cage fine. I went into the kitchen and when i came back i found the dumbo rat lying on her side moving her limbs and tail in slow motion, at one point her back legs seemed to twitch/scratch really fast and within a couple of minutes she stopped breathing!  She seemed really well before and showed no lumps or breathing problems.  Do you have any ideas what it could have been caused by!?
Thank you

Answer
She more than likely had a heart attack. This is not unheard of in rats and in fact, I had a pet rat die after play time right in front of me and it was a heart attack as per the necropsy.

This is usually a cause of sudden death.

I am sorry for your loss.