
I agree that supplementing the pup will interfere with milk production. Try weighing the pup before and after a feed, that should give some indication as to how much milk is being taken in.
I would be supplementing Mum to improve the quality and quantity of her milk, give goats milk to drink as much as she will take, other calcium rich foods,yoghurt,cheddar/cottage cheese,vanilla icecream etc.
Do you have Vet/Nutrodrops, they give lots of calories without filling her up? I get them to eat all sorts of foods, chicken, lamb including any juices given off during cooking, pilchards, basically anything she will eat that is OK for dogs.
Puppy Kibble should always be available for Mum as it can be left out in hot weather when other foods can't.
If there is no improvement in 24 hrs then a trip to the vet may be needed to check the pup over and that it is not becoming dehydrated.
Keep us posted