implantation bleeding or menstrual bleeding

so I took a plan b before ovulating , which was a week after my cycle . My fertile window closed on the 7th. On the 8th I experienced medium brown discharge, but on the 9th that discharge turned into red blood. The blood was not brown or light pink (it resembled my regular period) , but the flow was lighter than usual. Today I still have the bleeding but it is only a few dark drops. should I be worried?