Hi , I've got one better. My toilet seat also has a butt shape stain in blue/purple but I also have that same hue in the bathtub. I have tried everything and it doesn't come off. I am not pregant and have no changes in my life. My husband and I are in our 50's and can't believe this is happening. I am a neat freak and keep things clean so this is driving me crazy, I can get a new seat but it's a new bath tub that can't be replaced. I can't be no one knows what causes this.