I am sorry to hear you are worried about Maggie. My name is Rebecca. I am a veterinarian and will do my best to help.
Can you take her temperature with a thermometer?
Do her gums look pink, or pale?
How are the puppies doing?
Shaking and panting in a dog that gave birth a few weeks ago may be signs of eclampsia, or milk fever. This is a condition where too much of the calcium in her body is going into the milk for the puppies, causing her blood calcium to drop too low.
When she was at the vet, did they do any blood tests?
Is she eating and drinking?
If this is eclampsia, it is an emergency and she should see a veterinarian today to have her calcium checked and get IV calcium if it is low. If you can take her temperature and it is high, 104 or higher, I suspect milk fever/eclampsia.