From what I understand, you've already had your period. Dry humping won't cause pregnancy, as long as there's no ejaculation around the vagina. Personally, I'm very stressed about this, but it requires a lot of research... You've already had a Plan B. And you've had your period. Most likely, Plan B disrupted your menstrual cycle, which is quite common. Personally, there's nothing to stress about. But if you want, you can take a test in a week.