I suspect the same, but I have read and the doctors told me that FIP can be triggered by a vaccine and the cat was supposedly a coronavirus carrier before the vaccine.
Nevertheless it is very hard for the doctors to pinpoint the actual virus or cause of all this.
I have a 5 months old cat (male), breed: Chartreaux, name: Charlie :(
I adopted him 2 months ago and i went with him for his first vaccine. All good, but my cat was kinda skinny and he ate not too much. I bought him vitamins and everything seemed better, but he was still on the skinny side...