Had sex 8.23. took plan b 8.25. positive test today. when did i conceive ?

If truly positive...: If a pregnancy test is truly positive, that means the baby embryo has already implanted. If the baby embryo has not implanted, then the pregnancy test would not be positive. Because implantation is ABOUT 7-10 days after fertilization (conception), these dates look good (unless one was already pregnant before 8/23). Nobody can tell the exact time of natural conception, as nobody can see it happen.