My Puppy Died of Parvo?

We noticed my energetics, joyful 3 month old Frenchie not being herself . Not being playful and not wanting to eat her treats. We knew something was not right with her so I took her to the vet. They tested her for parvo and she came back positive. She stayed overnight ant the vet and they gave her fluids and medicine. I picked up the next day and she was still not doing well, but walking and happy to see us. The following day K gave her her medicines but she did not want to eat so I took her back to the vet. They gave me fluids to give her through an IV every 8 to 12 hours plus more medicine. She looked like she was miserable. But she was still was standing. It looked like she would rather be standing then laying down. She had diarrhea and would puke up clear stuff.

That day that she passed away, which was like four days after testing positive, I gave her her medicines and fluids and even fed her some food through a syringe just a little bit. She was still walking. To me I felt like she was doing a little better than the day before . I went back to my home office and like 30-45 mins later my daughter screaming 'mom mom Khloe passed away!' I was in total shock and misbelief. I had just seen her up and walking. I feel like maybe she had like a heart attack. What could have happened? We are so heartbroken.