Litterbox fiasco

So, I like to keep my people guessing. Lately, I've been not so inclined to poop in my litter box. Instead, I go on the rug right outside it. It's not that the box is dirty or anything. And I still pee in the box. My people got me new litter and a new cat food for "sensitive systems." And my person keeps looking at my butt and watching me every time I go down the hall. She said I might have to go to the v-e-t. That sounds bad. Maybe I'll feel like pooping in my box tomorrow.