I have a 4 week old puppy that is not sucking milk very well. It is getting smaller than the littermates and acting weak. What can I do?

Please Help. We have four week old puppies. The runt of the litter developed in the past few days tremors, has separated herself from the pack, does not want to be held but will still nurse from mom but for much shorter periods. She is still much smaller than the other pups. We had in the beginning made sure she was getting her own private feedings. We thought she was doing well. She was playful. Now she lays be herself. She does still urinate but we can't check the consistency of her stool because her mother cleans her. At first we thought that she was just chilly so we warmed up the room. Then we thought hypoglycemia but she fed and tremors remain. What do we do??? Your immediate response would be greatly appreciated.

Thank You,
Paul and Christine