Good luck with the new vet. Hopefully it is just stress from a combination of multiple vaccines and being in heat at such a young age.
There isn't a test for FIP. There is one for FCoV, but most cats have been exposed to it so test positive. There are tests for FeLV and FIV - were those the ones you were thinking of? Regardless, she should have been quarantened for a couple of weeks incase she had some sort of infection.
Being when she came from, un known cercumstances, she should have been quarentined, away from Baby, until she could be tested for FIP.