Niddah Nowadays

I have learnt through reading Talmud that in the time of the Talmud women waited just the days of where she is Niddah (still bleeding) then one clean day, then going to the Mikvah at night. However the Halacha Midrabanan added the seven clean days on top of the Deoraita. My question is first, who is responsible for adding the seven clean days?(which period , which rabbi) And are the seven clean days considered a Chumra Midrabanan? What is the position of some Rishonim? If we pass the ovulation period then we pass on a Mitzvah Deoraita (Pru Urvu) for a Chumra Derbanan... Please advise each reference when answering.

Shalom Uveracha, The Torah speaks about seven days from the day of ovulation until going to the mikvah. However in several places the Gemara clearly states that all Jewish women accepted upon themselves that they would act like zivah, i.e. seven clean days, even after seeing a small amount of blood. All opinions accept that since today we have to assume that women who see blood are in the category of zivah, then the seven days is din and not just chumrah. Kol Tuv