I Started My Birth Control A Week Later Then I

I started my birth control a week later then I was supposed to begin but had a period during that month. This last pack of pills I finished did not yield a period during the reminder pills. I am now three weeks late and have been cramping on and off for almost a week. I also suffered from a silent migraine for the first time ever. I have taken two pregnancy test about a week ago both negative. Could I be pregnant or did I just mess my cycle up with the week late pills?