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When calculating start of the period, any staining that precedes the period, while it may cause niddah status, will only count as start of period if it is accompanied by hargasha, the usual feeling that you feel when your period has actually begun. Cramps and other PMS symptoms are not hargasha. If you are uncertain, please consult your rav.
Any Veses HaChodesh dates on one's calendar that are passed clean are canceled (Rabbanei Chabad).
Kitzur Dinei Taharah 8:13 (Kehos) and other Chabad Rabbanim maintain that Veses HaChodesh date/s that fall within the days of the flow are carried forward until they are passed clean.
Rav Yekusiel Farkash, in the sefer Tahara K'Halacha, and other Chabad Rabbanim, maintain that any Veses HaChodesh date/s that fall within the days of the flow are not carried forward and are canceled.