My cat has been sitting in the room with her litter box?

Last night I had a very bad night, and the cat saw me upset. Since then, she seems apprehensive of me, and the last couple of hours, I've notice she is just sitting in her bathroom, staring at her litter box. I cleaned it thinking maybe that's the problem, but she is still doing it. Any ideas? She seems to be eating and drinking ok, in fact she just ate. What gives?