hello maikki
oh i am so sorry for you that your tabitha has died. :-(
it is a very sad thing when you lose a beloved pet of many years, & so suddenly too.
i am wondering if she contracted this FIP from your shoes or clothes, as they could come in contact with this bacteria/virus from the outside from touching other cats, or sitting where other cats have been? or could it be from the food?
i would go to the vet to have sluggo blood tested, just in case, & maybe he can then get vaccinated against FIP.
sincerely silkhobby