Replacing Litter Box. Your cat may be in labor. Cleaning the box itself regularly will help you get more life out of the litter box and keep it from getting crusty (gross!) tips for keeping your litter box clean.
If you clean and maintain it regularly, the box should last several years or more. If you are simply changing the litter box to the same size and shape as the old one, you could probably sneakily swap them out without your cat even realizing it. Washing is the part of litter box maintenance that people overlook the most.