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!