Taking an emergency contraceptive, like Plan B, can change your regular bleeding pattern. The Plan B pill is a dose of progesterone, a natural hormone that builds up the lining of the uterus. The medication is actually tricking your body into thinking it should have a period. You should have some sort of bleeding within three weeks of taking the Plan B pill. If you do not have any bleeding or spotting (minimal bleeding) within three weeks, visit Teen Clinic for a free or low-cost pregnancy test.