The mom and pups should have had a check up within the first week of giving birth to ensure both her and her pups' health. This will also ensure that on exam, there are no pups remaining inside of her and she does not need a c-section etc. Only reason for mentioning this is that if there is a puppy still inside, she would progressively become more and more sick, although I would have expected to show symptoms about 2 weeks ago. I would still recommend you speak to your Veterinarian to have her checked out for a possible mastitis - due to milk buildup in her mammary glands that are filling up with milk and becoming infected, as well as have a poop sample checked for parasites.
Even if these are not one of the causes, I would check her out to be on the safe side. Severe complications can occur at any time although it is not impossible that appetite loss and lethargy may be as a result of hormonal change.
Martin Slome DVM
Centre Street Animal Hospital
7700 Bathurst Street Units, 40-42
Tel. (905) 771-9855