Puppy died and had fluid in lungs.

Hi Dr. Marie,
My case is probally irrelevant now since my 14 weeks puppy(american bully breed) just passed away all of sudden. I wasnt sastified with the explainations from the vet and animal hospital gave me.He was perfectly fine until yesterday, i noticed his breathing was very strange like he struggled to breath and he didnt seem to respond at all. I called up the vet nearby to check him up and all the vet gave me was internal bleeding, but from what? The puppy didnt have any kind of accidents or abused! Since the vet didnt help my puppy in any ways. I took him to the animal hospital, by the time i reached the hospital my puppy started to have seizures, they tried to help my puppy but after couple of hours his nose started to bleed and he was choking on something, the doctors at the hospital told me that they did everything they could and the puppy will live no longer than an hour or 2. I couldnt stand and watch him suffer so i asked them to put him to sleep. I asked them about what were the problems. They told me that they need to do an autospy to find out. I agreed, a week later they called and told me that his lungs were filled with water. I wasnt sure if that really caused his death.
Im just trying to find out the reason! Im very grateful if you can help me out
Thank you very much,
Quan Doan