Did you start the pill within the first 5 days of your period? If you did then you were protected immediately, otherwise you’d have to wait 7 days. Your old cycle doesn’t matter anymore because you no longer have a cycle on birth control. The “period” you get is fake and it’s completely irrelevant whether you bleed or not. You also have a 3-6 month adjustment period. You took a pregnancy test well after 3 weeks from the unprotected sex so it seems like the plan b worked. Next time be more responsible and don’t have condomless sex until you’re protected by the pill and until you and your partner have been tested for STI’s