The reason for calcium AFTER whelp is to keep eclampsia at bay. Tums has plenty enough to help. they do not need a bunch unless they actually get eclampsia, and they don't need a bunch of supplements for prevention.
Severe panting is a classic sign of it, but it normally happens around 4 weeks after whelping.. Mastitis is NOT milk fever, eclampsia is known as "milk fever"...
The best way to prevent eclampsia is to
avoid calcium supplementation during pregnancy and to feed the pregnant bitch a well-balanced, good quality food. Supplementation of the bitch with calcium may be helpful once the puppies are delivered and are beginning to nurse. Supplemental feeding of the puppies may also be beneficial, especially for large litters.
Giving extra calcium during pregnancy is what causes eclampsia. I never give my girls cottage cheese or yogurt during pregnancy.
Read about it here... The first one talks about using TUMS:
Tums for the Mommy Eclampsia in Dogs Canine Eclampsia... Hypocalcemia... Puerperal tetany... Milk Fever in Dogs .... Chinaroad Lowchens of Australia