Is my action correct after finding moisture with an اثر (trace/mark) in my underwear during menstruation? Is this moisture considered ritually impure (نجسة) yet excusable (معفواً عنها), given that the impurity might have transferred to my body?

Yellowish or brownish discharge during the menstrual period is considered menstrual blood and is ritually impure (najis). As for ordinary discharges, they are ritually pure (tahir) according to the more preponderant opinion. If there is doubt about the transfer of impurity from clothing to the body, such doubt should be disregarded. If the transfer of impurity to the body is certain, it is obligatory to remove it by pouring water over the impure area, while also cautioning against obsessive doubts (waswas).