Peaches used to poop in the tub when I first got her. I took her to the vet and they tested her but nothing came back. She did have a bit of the runs at the same time. The vet recommended switching her to a premium food (IAMS or Science diet). That did the trick.
I agree. Best if you take the cat to the vet just to be safe.
When cats pee or poop in the sink or tub it is almost always a sign of physical illness. Please take the cat to the vet ASAP and carefully describe the behavior to the vet so she will know what to look for.
AFTER you return from the vet, you can try clicking here to help solve the problem. But, please, please, please take your cat to the vet before you try any of the suggestions on this link.