My period lasts for 7 days but now its only last 3 days and im not on any birth control i also took a plan b the morning after we had sex why is my period short ?

About Plan B...: Plan B can alter the menstrual bleeding for the cycle in which Plan B was taken, and sometimes the next cycle too. One should be sure to check a home pregnancy test about 3 weeks after sex, because Plan B works most of the time, but not all the time. That said, couples having sex should always be ready for a rare, but cute, surprise baby... and being married, settled down, raising a family,...