My dog was neutered yesterday & he died today?

I can't breathe. I don't know how this could've happened. Someone please tell me if you know anything about this. He's a pug & I had a meeting with the doctor prior to the surgery to make sure he knew how to deal with pugs, the anaesthetic, their breathing issues, the pallet, etc. and he assured me he has operated on many pugs & bulldogs. His breathing seemed a bit labored so we took shifts & stayed up all night with him and then this morning all of a sudden his hair was standing up on his back & he was biting his toung & barely breathing. We rushed him to the doctor which is all of 3 minutes away & they couldn't save him. They said he had fluid in his lungs & it could only have happened from an electric shock or a bug bite. He was inside all night with us. Then he said maybe his heart was bad but we did all the preoperative testing & bloodwork & the doctor even commented how well he was taken care of & then he had a strong heartbeat and everything seemed great, almost surprised him actually. And then this happens??? How else could a dog get fluid in his lungs? I'm so confused & lost right now. He was the most special dog I've ever had in my life. My husband is beyond upset about it. He's in complete shock & doesn't know what to do either. How could this have happened???