Do you have her and her litter in a quiet area, where she has privacy? Are there any other pets or kids disturbing her?
Is the litter box kept very clean, and close enough to the kittens so that she can get to it very easily?
Your cat can go into heat and get pregnant again before the kittens are even weaned, so make sure she does NOT get out.
You can spay her as soon as the kittens are weaned and her milk dries up.