31 days is the average gestation for a rabbit, though they can go a few days either way. Most does don't do the nest building and fur pulling thing until right before they kindle, though even that isn't a "given". I had a Jersey Woolly doe that started fur pulling a week before her kits were due, by the time they arrived, she was nearly bald!
Once the doe goes into labor, the process of delivery usually goes pretty quickly - the whole litter arrives in 30 minutes or less (that's a big litter, dwarfs have such small litters it takes a lot less time, of course).
Are you seeing any sign of blood, or a bloody discharge? That could indicate that she has a kit stuck in transit. Sometimes, does experience a false pregnancy, and will go through the whole performance of nest making, etc, and produce nothing. If you don't see babies within a week of this nest making episode, my guess would be that this is what she's done.