Why would a dog have fast breathing after giving birth two days ago?

She should be checked by your Veterinarian ASAP and I cannot stress this enough, immediately! She could be running a fever, normal temp is 101 - 102 degrees. She possibly could have a retained pup inside of her that is starting to decay, she could have ecplampsia (deficiency of calcium that is very dangerous & could cause death fast), a retained after birth, etc. The most important thing is to get her to the Vet immediately.