Hey if you have a pregnant rabbit do the following things:
1) make sure that she is in a large cage, with babysaver wire, alone.
2) keep her in a quiet relaxed atmosphere so she doesn't stress during her gestation period of 31 days.
3) make sure she gets the proper nutrition of about 1/2 cup of alfalfa rabbit pellets, timothy hay, and plenty of fresh water!
Count 31 days from the date bred for the babies to be due. Put the nest box in about 5 days before she is due, and if she is going to have babies you will see her pulling fur a day or two before they are born to prepare.
The nest box should be made of wood and protected from draft and rain. If the box is wood, it won't conduct the heat and cold. The box should have 6 in. of pine shavings in the bottom and fluffy timothy hay on the top.
Oh, and one final thought: Are you sure your bunny isn't just fat? Usually, it is quite hard to tell if you have a pregnant rabbit.