R.K.
If your friend is really a friend she will understand. Call her and say I feel horrible, you know how I get when I'm pregnant. I'm feel like I'm gonna hurl at any moment, I'm tired, hubby's going out of town and the babysitter is booked. And what I need right now is a day off to sit and watch a movie and take a nap. So sorry if I am gumpy.
If she is really a good friend she will say something like 'can the kids come over to my house tomorrow' or 'can I bring you something' or 'I can come over tomorrow and help with the kids and you can rest'.
Friends understand and help when they can. And if we have been unusually grumpy or snippy they know something is up and love us anyway -- as we would for them.