That first post is totally false! Yes, your period flushes out your system, but if he came in you and the next day you start your period, you are flushing out the things you collected in your system throughout the previous month. You can get pregnant at any time during the month, including the day before, during and the day after your period. Anyone who tells you any differently isn't informed correctly.
So, if he came in you today and you got your period tomorrow, if you start ovulating right after your period and somehow some sperm was still in your system, you could get pregnant.