On bcp apr-sept, last active pill sept 13. 1st day of bleeding sept18, unprotected sex sept21. period ended sept22. period again sept 28. pregnant?

Unlikely: It's unlikely that you got pregnant on 9/21, but I typically tell women there's no perfectly "safe" day. 10-15 days from 9/21 you would have a positive pregnancy test if you were pregnant, so you can check in October. For an average woman, your most fertile days are days 9-14, with Day #1 being the first day of bleeding. Hope that helps.