1. A woman is considered niddah only due to uterine bleeding. Our Rabbis of blessed memory learned this from the verse "And she exposed the source of her blood" (Vayiqra 20:18), from here it is learned that a woman is only niddah from blood which comes from the uterus**. Therefore, blood which comes from the Fallopian Tubes or from the sides of the vagina does not make the woman niddah.
2. According to the Torah, the status of niddah (and all the restrictions attendant upon such status) obtains when the woman felt herself menstruating, for example, if she felt her body shake and then she saw that she was bleeding, or if she felt her cervix open and she found blood upon an internal examination.
If the woman saw blood but had neither of these feelings she is still rabbinically considered to be niddah. When consulting with the rabbi it is often important to inform him if there was such a feeling.
3. If a woman had such a feeling she is niddah from the moment the blood enters the vagina even though it has not left the body.
4. "The daughters of Israel" have taken upon themselves an extra stringency concerning niddah, namely, that they will wait "seven clean days" before performing the ritual immersion, even if only a drop of blood was seen. (According to the bible the law of the "seven clean days" does not apply to a woman with normal menstruation, only to a pathological situation known as zabah (gedolah). Nevertheless, in our days the "seven clean days" are counted before immersion even for a normal menstruation.)
The woman remains in her niddah status until she checks herself making sure she is no longer bleeding, counts "seven clean days," and ritually immerses in the waters of a kosher mikveh. It is absolutely forbidden to ritually immerse before the end of the seven clean days. (In a case where the woman consistently ovulates during her seven clean days and cannot become pregnant unless she immerses early, the couple should consult their rabbi.)
* The laws of this chapter are adapted from Taharat Habayit Vol. 1 Chapter 1.
** In Hebrew the word for uterus and source is the same: "Makor"