Nursing is hard work, and can cause heavy breathing as she adjusts; however, the panting is often caused by low calcium, which can be very dangerous and cause seizures and even death. So give her a dish of kitten formula every day. If you have none on hand, give her some milk right away. She needs the calcium and the milk should help, but you’ll need to buy a can of powdered PetAg kitten formula ASAP. Cow’s milk can cause diarrhea as cats are lactose intolerant.