Do you live with her? How does she know your schedule? I feel its none of her business, and would tell her politely that you are going to listen to what your body, and baby need.
I know the feeling I lived with my mother in law my last pregnancy and she expected me to clean cook and everything when I didn't do anything but sleep. She always accused me of cheating on my fiancé and said I was not fit to raise a child. She kicked me and my son out of her house when he was just 3 weeks old and I had to have all of our things out that day. So needless to say I don't like her at all but I put up with her because that is my sons grandmother (unfortunately) lately she's been trying to kiss my *** to get more time with him and she's not happy because me and my fiancé just got our own place so now he will be moving out of his mothers house and me and my son will be moving out of my mothers house. Either way, do what you feel you need to do, right now your body needs all the rest it can get because once your bundle of joy comes you won't be getting any sleep.