Irregular cycle after removing patch

2 months ago, I went off the birth control patch, my first period off the patch came at the normal time, but I am over a week late for this one. Could this be due to coming off birth control even though my first period off was normal? or could I be pregnant? I have taken 2 negative pregnancy tests this week, however, I am worried I may have tested to early...the last time I had sex was monday. Dec.21st (sorry if that is too much info) but I've been reading online that thier needs to be a certain amount of time for conception/implantation before tests are accurate??? Does anybody know anything about this???