Why did my puppy die?

my 10 week old puppy died yesterday, she woke up at 8am and she was her usual jumpy self but by 9am she suddenly changed, she struggled to balance and her eyes start to glare. she tried to carry on playing but ended up collapsing on my sofa, we called the vets and rushed her in by that time she lost control of her bodily functions and the fecies gave out a horrid stench. soon as we got to the vets they said she has mouth ulcers and put her down there and then saying it was parvo. i did some research and parvo does not kill a dog within 45 minuites. i was told she might of been poisened but was impossible as all proper precautiouns were taken care of and she was shut in my bedroom with me which has nothing harmful to pets. i'd hope someone could tell me how my puppy died which i hand reared from birth because the mother didnt take to her.