What happened to my puppy?

I have 10 puppies that are a little over 6-weeks old. Last night we fed and watered them right before bed and they were ALL playful and ate hearty and drank their water. That was around 8 or 9pm. All of a sudden at around 11 or 12, one of the puppies was making a horrible noise, blood was coming out of his mouth, and puke was just pouring out. I called the vet and they couldn't tell me what was wrong but said hes not going to make it. He died soon after :-( Any ideas of what could have happened. I separated the other pups and they are still doing great at 6 this morning. I'm so worried about them