Your rabbit may also be marking when she stops using her litterbox and begins leaving more droppings around the house, to stake out a perimeter. In females you may first see a noticeable increase in aggression. She is guarding her nest, even if she isn't pregnant. Two rabbits who may have gotten along fine until this age may begin fighting for dominance. This can happen even among siblings.
Females sometimes have a greater penchant for digging.
While digging should be expected of any rabbit, an unspayed female may be particularly energetic. Unfortunately, she may dig your carpet or burrow out of the yard to find a mate. The transformation won't be immediate. For the house rabbit, behavior and livability in a human household may be more pressing considerations. Additionally, introducing two unaltered rabbits could result in serious injury or death as they fight for dominance.