Can you explain a little more what you mean by a hard ball? Is her abdomen (tummy) swollen or is it one of the nipples or is it down near her vent? What is the size of it?
1. If its her tummy there could be an infection inside (pyometra) or a retained pup that she has not been able to give birth to.
2. If it is a nipple it could be mastitis or a blockage in the milk ducts or possibly because she's just started suckling the babies and they may have been a little rough.
3. If it is near her vent (where she gave birth from) it could be a prolapsed uterus or a large pup that has become stuck.
Unless we are worried about our mums we don't handle them so soon after birth so I can't really say what the nipples are like but any of the other conditions would definitely require veterinary attention which may mean that the mother will not keep the pups due to disturbance. I would make sure that I discussed it fully with the vet before risking moving her. I don't not think that the pups would survive away from the mother unless your vet was a very short distance away and you would have to take both mother and pups IN the nursery cage, I would definitely not take them out of the cage. Hope she is OK and the pups too. We will look out for how you get on
.