My cat also pees in the bath when she gets cystitis. She doesn't have kitty litter though - fully outdoor pee/pooper.

My last cat used to hate litter that wasn't totally fresh each time she used it (crystals weren't around then - i have a feeling they may have suited her better) and she used the bath, the shower or the bathroom sink or the roof gutter because it would drain away entirely. She was a bit eccentric though, hygiene was an absolute priority for her. Every activity she did was preceeded and ended with a thorough wash all over!!

Vet check really is the most important thing at this stage to rule out any health issues before proceeding on behavioural issues. I hope it is just the change in her environment and/or kitty litter change.