
Has she got the option of going somewhere cool if she is too hot? If the pups are on a heat pad there should be an area without that both Mum and pups can migrate to if things get too warm.
Normally panting is connected to the uterus contracting as the pups feed causing pain, if I remember rightly she was spayed so that won't be the cause.
Never had a bitch with low calcium[eclampsia] because I give calcium rich foods, I know the eventually have seizures but don't know if panting happens in the early stages.
Hopefully it is just a phase she is going through.