Sounds like she did have adequate vaccination protection, so may not be Parvo. However to answer your question, yes Parvo can be fatal in a matter of days or even hours.
As for what else this could have been - would be pure speculation on our part.
I am so sorry this happened to your very young and loved dog. Talk with your veterinarian about sanitizing your environment well before bringing in another pup. And just as you did before, get all those shots! Remember, vaccination is our best attempt to prevent
animalsAnimal shape vitamins
Animal shape vitamins with iron (and humans) from getting disease. BUT NO vaccine is 100% effective. So much depends on chemistry, virology, bacteriology, immunity, dose of infectious agent etc. Science is still a mystery.
Kimberly Coyner, DVM DACVD