Well, generally conception (meaning, sperm entering egg) doesn’t occur until hours after pregnancy, sometimes DAYS. After that, it would be between six and fifteen days until the embryo implanted, which may or may not cause light bleeding. If you had a full-on period the day after sex, that was heavy or lasted eight days, chances are, you’re not pregnant. You actually lose the uterine lining several days after the egg is expelled (yes, the egg is expelled from the body if you don’t get pregnant, because you don’t need it, so it goes out the same way your period does), so you probably didn’t have an egg waiting to be fertilized, and it would be some time after the end of the period that another one would be released. Don’t worry. Wait until the time comes for your next period, and if you miss it, take a pregnancy test. Whatever you do, don’t stress out. It’s bad for your health, and if you’re prego, it’s bad for the baby, too.