What is the ruling on very light spots of menstrual blood that appear on outer pants due to excessive wetness and water during istinja’ (cleansing after urinating or defecating), and is the place where the girl sits considered impure because of this trace?

Many scholars forgive a small amount of blood, even if it is menstrual blood, so the light spots present do not harm. A small amount of impurity is also forgiven. When in doubt about the transfer of impurity, the original principle is that it has not transferred. One must abandon obsessive thoughts and not pay attention to them.