Maybe she's scared for some reason - has something changed in your bathroom? It might be possible that she's heard something and interprated it in a funny way so maybe you could explain the plumbing to her in a very simple way and emphasise that it only goes one way just in case she thinks that there's a pee gobbler down there or similar.
If she had a UTI you could maybe take her to the Dr just to check that it has cleared up properly.
Good luck. I hope you find a solution soon.