What is going on with my cat?

We took in a cat that was supposed to be spayed, but low and behold - she wasn't and recently had kittens. The kittens are now 6 weeks old and we've easily been finding homes, but ever since she had them she will not use her litter box. She is going absolutely EVERYWHERE and we don't know what to do. This has not happened in the entire year that we've had her. I love her and it would break my heart if we had to give her away, but my boyfriend is furious with having to constantly clean this up. We've had to throw away a bed sheet, a rug and even some clothes. Help, please? Is she sick? We will take her to the vet asap, but would like some opinions..