Help

My orphaned puppies of 2 and a half weeks is getting sick and die one by one. It happens 1 every 3 days. They start by being very healthy and suddenly after a feeding one would get kramps (it looks) and make sounds to let you know they have pain. They stop eating and becomes very weak. I took them to the vet when the first one started and they all got de wormed. I thought it was constipation and gave a ennama of 3-5 ml luke warm water but pain doesn't go away. I force feed them when week with one part glucose silution, one part rehiderate and one part formula with a syringe but they die. I keep them warm enough and feed them puppy formula. What's wrong with them and what can I do to help them when they get sick.