In my experience, a cat that changes it's behavior regarding pee and poop is trying to communicate something. I would pay extra attention and consider a visit to the vet.

It could also be that she just saw you watering plants and thought, why not. My cats usually go outside to poop or pee and just eventually use the sandbox. When they pee outside, they go for the sinks on the garden and when inside, they sometimes use the WC.