Sitting in the Litter Box?

She's never done this before, but as soon as Shinobi started going into labor, she wanted to lie in her litter box. I took it out of the area while she was giving birth (didn't want to risk infections), and put it back in after she was done, since she was in a cage overnight. I found her this morning back in the litter box with three of the kittens. What does that mean? I've since moved them all to a box with high walls, but what's with her and the litter?

My oldest cat does this sometimes when she wants to hide or when something signigicant has changed for her. We've always the kept the litter box with a lid on top in a separate closet so I guess she goes in there to feel safe/hide. I have two cat houses, space under beds, other bedrooms, closets, etc. but she has always preferred it.