A Little While Back, January 2nd I Believe, My Boyfriend

A little while back, January 2nd I believe, my boyfriend touched my vagina with a little bit of semen on it, though it had pretty much dried at that point. The next day I took a Plan B One Step. A week later I got what seemed to be my period, (normal flow, 5 or 6 days) is that normal? The time where my regular period would have come already passed. Did it reformat to the after plan B period? Should I worry?