It saddens me to read about your loss.
My angle on this is, you took the puppy to the vet, and obviously the vet found the puppy healthy. Hours later, after being treated, the puppy was gone.
Perhaps there was something wrong beforehand, that the vet did not find (unlikely), but there is always a possibility. We are talking about a young puppy. But then, what if the vet did make a mistake?
I am concerned about what sort of response you did receive from the vet when you told them that the puppy had passed away. I am also concerned about whether or not they asked you to bring it in so they could determine what had happened to it.
I am also having a difficult time understanding why the vet did not tell you to bring the puppy back in when you called with your concerns. After several calls, I would think that they would have taken notice and been concerned themselves.
I am not sure I would sue, but I would persue for more information.
Please let us know how all of this works out for you. I hope you can find peace again (no depression) soon.
Mary, Samson, Delilah, and Tyra