Rabbits will do this for various reasons, but the most common reason is a pseudo pregnancy. It’s when an unspayed female has hormonal signals and it makes her think she is pregnant. These hormones will make her go through all of the motions of getting ready for a litter, including plucking fur and building a nest. And yes, they can fit A LOT of fur in their mouth. Assuming that she is unspayed and hasn’t been around any males, then that is most likely what is going on here. The best thing you can do is let her build the nest. Otherwise she will be stressing over not being able to. After awhile, this will pass. Having her spayed will prevent her from going through this repeatedly, which can be very stressful for some bunnies.
One of my boys would “nest” when he was stressed. You wouldn’t believe the ball of fur that came out of his mouth. He never actually built a nest or anything. He would just run around frantically with fur and hay in his mouth. This would only last an hour or so. With girls and psuedo pregnancies, it can take days or longer for them to snap out of it.