Runt of the litter

Question
We have a puppy born April 13th. She is very small (I mean very small) compared to her other litter mates. At 12 weeks she weighed 3.2lbs. Is it possible for an adult female already pregnant to get pregnant again.

Answer
I think I figured out what you're asking. Are you asking if a female dog can become pregnant by more than one male? If so, then the answer is yes, but all the males must mate with her during the time she is most fertile, and even then, there's no guarantee as to how many puppies are fathered by which male.

What breed is your puppy, and how big are her littermates? What and how much are you feeding her?