I'm so sorry for your loss.
Maybe she has eaten something that was poisonous for her, rat poison, weedkiller (eg from an infected puddle) insecticide, slug pellets, anti-freeze or something like that?
She could also have eaten a rat or mouse or something that had been poisoned.
Sometimes they can swallow stinging things like wasps or bees.
Could ther have been any drugs or similar lying around?
Could she have been hit by a car round her stomach area?
There are some ideas here - I thought twice about posting as it's what you can do for a cat who has been poisoned but it seemed like there was nothing you could do...
http://www.doctordog.com/catbook/catpoison.htm l
Could you give the local vet a ring and see if they have an idea? Did see see a vet before or after she died as they may have more of an idea.