Cat that just recently had kittens?

My cat had some kittens on the 4th of July. She is doing the normal with them. She just came out to drink water and she is breathing as if she is congested. I have never seen her do this before. Is this something that I should be concerned with? She has food in the room as well as water and I understand that it is alot on the cat to nurse kittens. Also one of the kittens died ( I think she sat on it during the night). I just want to make sure I dont have to rush her to the vet. She seems to be tired which is understandable. Any suggestion would be helpful. Thanks