Maikki, it is probably that Tabitha was born with it as are alot of cats, they can be carriers their whole lives and not ever show signs of the disease being active and thus live normal lives. Thinks can trigger FIP such as stress on the animal, this will result in a quick and painful ending which leaves the owner wondering like you are.

As I said most animals are born with it and never have it effect them and others are not so lucky. The only way to find out if they are carriers is to get them checked for it before they get their vaccines against it as if the cat does have it then the vaccine will be useless.

I have done alot of research on this and it is a terrible disease to have. There have been rare cases where the animal HAS lived past the first bout (it will show up again tho) but only to take it's toll the next round. You did the right thing in what you decided to do as by the sounds of it she would not have lasted the night or next day.

Hope this helps you

Shar xxx