Siman 183
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א אִשָּׁה שֶׁיָּצָא דָּם מִמְּקוֹרָהּ, בֵּין בְּאֹנֶס בֵּין בְּרָצוֹן, טְמֵאָה, וְהוּא שֶׁתַּרְגִּישׁ בִּיצִיאָתוֹ. וּמִיהוּ, מִשֶּׁתַּרְגִּישׁ בּוֹ שֶׁנֶּעֱקַר מִמְּקוֹמוֹ וְיָצָא, טְמֵאָה אַף עַל פִּי שֶׁלֹּא יָצָא לַחוּץ, וַאֲפִלּוּ לֹא רָאֲתָה אֶלָּא טִפַּת דָּם כְּחַרְדָּל, יוֹשֶׁבֶת עָלָיו שִׁבְעָה נְקִיִּים. הַגָּה: כַּאֲשֶׁר יִתְבָּאֵר מִשְׁפָּטָן לְקַמָּן סִימָן קצ"ו. וְאֵין חִלּוּק בֵּין פְּנוּיָה לִנְשׂוּאָה לְעִנְיַן אִסּוּר נִדָּה (ריב"ש סִימָן תכ"ה מְבִיאוֹ בֵּית יוֹסֵף), כִּי כָּל הַבָּא עַל הַנִּדָּה חַיָּב כָּרֵת.
1 A woman who has a discharge of blood from her uterus, whether due to outside factors or purposefully, is tameh (impure) if she felt it flowing out. If she felt the discharge she is tameh even if the blood did not flow out of her body, and even if she only saw a drop of blood the size of a mustard seed; she must observe seven clean days for it Rema: as will be explained in Siman 196. And there is no distinction between a married and an unmarried woman with regards to the laws of niddah (Rivash in Siman 425 as brought by Bet Yosef), because all who have intercourse with a niddah are liable for karet.