Uterine bleeding can make a woman niddah, even when it is not, medically speaking, menstrual blood. Whether or not the bleeding you describe made you niddah depends on some details. Provided you did not have a hargashah, staining on colored underwear does not make you niddah. However, if you have a flow of blood (akin to your lightest periods) or have seen blood leaving your body, you are in niddah. Blood stains on the lower part of your body also render you niddah if they are beyond the size of a gris (roughly, the size of an Israeli shekel or American dime).
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