I know from a rather unfortunate experience that a doe can get pregnant almost immediately after giving birth, as this happened to a rabbit I rescued. Happily, mom, dad and both litters (a total of 16 kits) all survived the ordeal, and continue to be healthy and happy 3 years later. It is recommended that you give a doe 1-3 weeks after weening a litter before trying to breed her again, but she can become pregnant much, MUCH sooner.