Unprotected sex 7 days after period end, and took plan b. 6 days later i am cramping/blood. side effect or implantation?

Not likely implanta.: 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. Be sure to have a Plan A (good contraception method) in place, so that Plan B is never needed again.