Ppowb has given you excellent insight. Plan B can make your period irregular for 2 to 3 full months after taking it. Here is an article as well that may interest you regarding determining if you are pregnant. Store it away for the future too. https://www.medhelp.org/pregnancy-health/articles/Could-You-Be-Pregnant/1081
We wish you the best of health!
4848, PlanB messes with your next period, as you discovered. It seems like it might have messed with the period after that, too. You are not listing any early pregnancy signs so I think you are in the clear. Run a home pregnancy test to ease your concerns, then just wait for your system to straighten itself out.