Will plan b affect my period i was supposed to have my period today am taking plan b tomorrow will my period be thrown off please someone help?

It can happen: 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. Plan B is called Plan B because it is not Plan A (if having sex and not wanting pregnancy, be sure to have a reliable Plan A such as condoms).