This may be due to nutritional drain from the pups or intestinal parasites adding to this drain.
Its really common for mother dogs to lose coat and condition while nursing.
One of the things you can do to help her is have your vet check a stool sample to see if she needs to be wormed. If she does her pups will likely also need worming at 2-3 weeks and then 4-5 weeks so discuss this with your vet.
Another thing you can do is take a wet facecloth and wipe her down well. Do not use soap or a flea shampoo just plain water and wipe her down as sometimes birth fluids on the skin from whelping will cause skin irritation.
And to help her with her nutrition a daily multivitamin tablet for dogs, adding goats milk to her diet, and making sure she has free access to food while nursing can help.
www.meyenberg.com may help you locate a retailer for goats milk and that helps a mother dog make great milk without draining her body so much.
Hope this helps you!
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