If you found a small stain on your white underwear, check to see if it is smaller than a gris. A stain of any shape, whose area is less than that of a US dime or an Israeli shekel (a circle 19mm in diameter, or about 2.8 square cm), does not invalidate your clean days.
If you're not sure of the area (e.g. it not perfectly round), or you found the brown spot on the moch itself or on a bedikah cloth, then the stain has to be shown to a rabbi in order to determine whether it invalidates your clean days. If it does, then you must do a new hefsek and start counting over again.