Thank you for reaching out to us.

A stain found a surface that is not susceptible to ritual impurity (e.g., plastic) does not make a woman niddah.

As you note, the toilet bowl is made from porcelain, a material that can be susceptible to ritual impurity. However, an object connected to the ground – like a part of a house, or a toilet that is permanently attached to the floor and/or wall of the house – is not susceptible to ritual impurity. Therefore, a stain on a toilet bowl does not render you niddah, even though the bowl is porcelain.

Furthermore, it sounds like the stain you saw was smaller than a gris (approximately the size of an Israeli shekel coin or a US dime), which is the minimum size for a stain to render a woman niddah.

Learn more about the halachot of stains here, and please let us know if we can be of further assistance.