>is it normal that Maisy is panting?
100% definitely YES. So don't worry. My girl sounds like a steam train, and last litter she was doing it for over a week. Remember how hard their little bodies are working now. Apart from the pain they are experiencing, they are working hard making all that milk.
All bitches will vary, but I already knew that Suzie was one of the exceptionally noisy ones.
Your vet will reassure you that there is nothing wrong. :-)