My rabbit has been eating a lot less. She also lays down a lot. Should I be concerned?

You should. A rabbit needs to eat to survive. It's not like us, or many other animals, where food is just something that sustains us. For a rabbit, if she doesn't eat, her digestive system will shut down, and she will die. They can never go more then 12 hours without food.

How old is your rabbit? Is she spayed? How much exercise does she get? What types of food are you feeding her? Does she get plenty of timothy hay, every day?