Call an emergency vet now. It could be parvo.
My puppy got it and not to scare you but it can kill puppies. Is there any blood in the poo? If there is then it is very serious. it is very scary but they can make it.

ETA- I should add that my puppy got it at the same age as you and the entire litter got it. I thought it must have been just something he ate as he was fine one day but they deteriate (sp?) quite quickly.

Is the puppy drinking any water or eating any food?