What happened

Question
We bought a 2-months yorkie (500g) on 3/8, an active and playful puppy. On
the 3 day vet told us that the puppy was in good condition and we needed to
visit him for the 2 vaccination in a week. (The puppy had his 1 vaccination
3/3/09 (canine distemper, parvo, influenza and so on, modified live virus).
We bought a new dry food for him at the vet's clinic next day and started to
mix it with his old food that we got from the shop. Following our vet's
recommendations we started to reduce his food intake from 5 to 3 times a
day. Everything seemed fine. On Fri he was very playful, ate quite good, on
Sat morning seemed the same. But then he started having rapid breath and
gagging like he wanted to vomit. It wasn't look very awful but we decided to
take him to the vet as we were very worried such a small creature. The vet
said that no fever and seemed stomach problems and injected an antibiotic
with glucose, said no water and food till next morning. Then we needed to
give him a medicine. We brought him home 5 pm and he almost immediately
had a first seizure. We kept him without water (!) and food. But it got worse
and worse, more seizures followed, gagging worse: it looked like rapid breath
and cough all together, his small body was trembling and shaking all the
time. We tried to go to another vet but he was closed already. In the night he
started to scream and cry. He died at 7 am on the way to another vet. What
happened? all family are in shock, we all are crying.. what did we do wrong?

Answer
I am really sorry to hear about this Anna. First of all, you didn't do anything wrong. I have no way of knowing what happened, but if you take his body to the vet and have a necropsy done you might get an answer to what happened.

In any case you need to call the breeder you got him from and let them know that you lost this puppy through no fault of your own.

Small dogs have some issues that might not show up until they are weaned such as seizures, brain problems, etc,. These are the things that can cause small little ones to fade away or have serious health issues.

However, I am not a vet, I am a vet tech, so you might want to re-post and ask one of the veterinarians here their opinion about what happened.
I am so sorry you are going through this. I know how terribly painful it is.
Please don't blame yourselves because I can tell you did the best you could and the only things you could do.

Please let me know what you find out with the necropsy.