One theory is that she had eaten something outdoors which made her sick. For example there's been warnings about a mushroom infested with a parasite which causes food poisoning if you eat it -- and Röskva is like Natasja in that she happily eat whatever mushroom she finds. Which of course isn't very smart and hard to teach her not to do. Mother says that in this case that couldn't've been the reason, as she watched out for it, but it could be some other kind of infection from what she finds when digging.

I don't much believe it could've been a disease she picked up this Saturday, although it's not impossible. Neither do I think a that much delayed stress reaction is to blame.