She is more than likely pregnant. At some point, close to delivery, she will begin "nesting" and she'll start collecting hay. I would count forward 26 days from the day she was with the male and then put some sort of nesting box in her cage. You can purchase one or you can use a cat litter box, but make sure you attach it to the cage somehow. When she has her babies, they will flutter around and could possibly escape the box, so you'll have to check on them and make sure they are always in the box. When you put the box in her cage, fill it with hay or shredded newspaper and mama will do the rest. When my rabbit had babies she thumped around the cage and it was very obvious that she was giving birth (surprise! 10 babies!). The most important thing is to check on them a lot to make sure they are in the box and eating from mom. Having a scale to weigh them is a good idea so you are sure they are nursing. It is very common to have a baby that doesn't survive so watch if she has any really little ones, as these will probably be the ones that perish. They are helpless when they're born but mom will take care of them. When they open their eyes and start jumping out of the nest box on their own, you can remove it but they will still need extra hay/bedding material to burrow into. You can handle them upon birth as long as mom doesn't get mad. I highly recommend handling them to make sure they are always warm. If you find one that's cold, slip it in a towel and warm it under your shirt and then put it back with the others. Baby bunnies are the cutest little things. You will be amazed at how quickly they grow!
Also, closer to delivery, she will start pulling her fur from her chest to build a nest. If you find it laying around, put it in the nest box. Her fur will keep the babies really warm. When you start to see the fur, she's probably close to delivery. For mine, she ran around the room she was in, tore open the hay bag, and started pulling massive amounts of hay into her cage. Once she'd gotten the hay in she started pulling out her fur and create a nice little nest for them. It was really funny to go from a completly clean cage to, 20 minutes later, a cage so full of hay and fur (especially because we had no idea she was pregnant). Buns are really good moms. Don't worry!