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I would take her immediately to a vet.
"Normal mother dogs are bright, alert and attentive to their puppies. She should have a ravenous appetite as she converts metabolites to milk. You should become concerned if the mother shows any signs of listlessness or depression." - http://www.2ndchance.info/caninelaborstages.htm
She may have another dead puppy inside of her that could be causing her to be sick and may be too small for you to feel. A vet can do an X-ray and examination to make sure there are no internal issues.