"There is a machlokes whether this is an issur d'oreisah (Ravad) or an issur d'Rabbanan (Tosfos)." (source)

"From the practical point of you, the nature of the issur of a woman remaining in her Nidah status does not change until she has gone to the mikveh and as known there are other stages along the way,among them the 'Shivah Nekiim' as you mentioned, before going to the Mikveh." (source)