Am I Preggo or not Preggo?

How effective are the Plan B pills if the first pill was taken on the last(3rd) day and the last pill taken after that? Im asking because I had Dec. 24 my partner d in me, 3 days later I took the first pill the 27th(3rd day), a week or two later I had a period(spotty at first & was due around that time), now Im having headaches, menstrual cramps, backaches from time to time. What could this be?