I am so sorry for your lose. At that age - their lungs are not fully developed. I am so sorry.
Make sure you have gotten rid of all the towels, bedding (wash) so she doesn't smell the pup. This is one of their highest sense and even if the pup is gone they do leave their scent for awhile. Also go and buy a small little toy - soft and small to take the place of her pup. Make sure you transfer the pup smell to it (rub against the blankets it was in. It may help the mama.
It will just take time. Dogs do grieve!
T.