Plan B works by delaying your ovulation. If you're already ovulating, then it can't do that. You'd need one of the other methods of emergency contraception, either the pill Ulipristal Acetate, or an IUD fitted by a doctor. Either of those things can be effective if done within 120hrs/5 days.
Most women ovulate around the middle of their menstrual cycle, so days 10-18 is a good guide of when Plan B may not work for you.
The 1st day of your cycle is the 1st day of your period.
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