Its quite rare for a dog especially a big dog to only produce one pup there is most likely another puppy/puppies or it can be an infection from the delivery take her to a vet asap have her checked out
My dog is a black lab mixed with something else. not sure what. she had one puppy two nights ago and it died the next morning. Today she is walking around the yard shaking, straining and pushing. what does this mean. was that the only puppy?