sometimes your periods are just off once and awhile. Maybe you have always had an 'off' cycle but just didn't really notice it since you weren't ttc at the time or charting and such. If it were implantation bleeding than it would differ from your normal period as far as constancy - like spotting and coloring. With cramping, I am sure it is a sign of implantation too...Just make a note of it and if you have anymore symptoms with being preg, test

When did you start your period??? I have read that implantation can occur between 5-12 days after fertilization. best of luck!