Premenstrual stains

Thank you for this wonderful service you provide!
More than a week before I expected to menstruate, I noticed a red stain on my underwear. Since I was wearing colored, I ignored it. This was on Shabbat, or possibly on Motzai Shabbat, I don't remember. On Sunday the same thing happened, but the stain was larger. Sunday night I stained again (on colored) & became suspicious that this could be my period, arriving early. I told my husband that I wasn't sure of my status & as a precaution we did not have relations. Monday morning I realized that I was beginning to stain heavily & that my period had begun. A few questions: Which day should I consider as my first niddah day for the purpose of making a "hefsek tahara" ? And from which point should I calculate my "onot prisha"?
Is there a rule of thumb you can share with me for this type of situation?

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