You are dealing with a very central issue. What is simple is always a bit wrong; but it may be good as an introduction, and you are going to understand why a secure common way has been setup by our teachers for all without exceptions and without possible recourse.
A first notion that we want clarify is the notion of close of the Talmud, see Rambam in the introduction to his Mishne Tora:
.. הוֹאִיל וְכָל אוֹתָם הַדְּבָרִים שֶׁבַּגְּמָרָא הִסְכִּימוּ עֲלֵיהֶם כָּל יִשְׂרָאֵל...
The entire people of Israel accepted everything is decided in Talmud.
An important point is a testimony of the Talmud (Nidda, 66A)
אמר ר' זירא בנות ישראל החמירו על עצמן שאפילו רואות טפת דם כחרדל יושבות עליה שבעה נקיים
R`Zera stated: The daughters of Israel have imposed upon themselves the restriction that even if they observe a drop of blood of the size of a mustard seed they wait on account of it seven clean days.(Soncino's translation)
and Brachot 31A
ת''ר אין עומדין להתפלל לא מתוך דין ולא מתוך דבר הלכה אלא מתוך הלכה פסוקה והיכי דמי הלכה פסוקה אמר אביי כי הא דר' זירא דאמר ר' זירא בנות ישראל החמירו על עצמן שאפילו רואות טיפת דם כחרדל יושבת עליה שבעה נקיים
Our Rabbis taught: A man should not stand up to say Tefillah either immediately after trying a case or immediately after a [discussion on a point of] halachah; but he may do so after a halachic decision which admits of no discussion.
What is an example of a halachic decision which admits of no discussion? - Abaye said: Such a one as the following of R`Zera; for R`Zera said: The daughters of Israel have undertaken to be so strict with themselves that if they see a drop of blood no bigger than a mustard seed they wait seven [clean] days after it.
After this important introduction,
We can examine in Parashat Metsora two kind of "uteral bleeding": the first is called Nidda and the second Zava. The first refers roughly to healthy menstruation, the second to an intermenstrual bleeding. Normal (healthy) bleeding ends naturally within 7 days. An illness ends when the woman heals. There is an alternation between Nidda and Ziva (intermenstrual bleeding) periods. We may lose count. A range of interrelated stringency will make us sure that there is no sexual intercourse in a banned period. In nidda the uncleanness is 7 days, after this, in the first evening after 7 days, immersion in mikve allows marital relations. In Zava gedola, a count of 7 days without bleeding is required, after this, in the middle of day 7, before the evening, immersion may permit marital relation. We have the custom to wait the evening because the original tevilat nidda is in evening (op cit).
If necessary we getting more technically into the substance of those laws, through Rashi in Megila 28B and rabeynu Yona in Rif Brachot 21B.