Period a week after plan b. Does that means I am not pregnant?

Probably not. Early shedding of the uterine lining will prevent implantation of the fertilized egg.Minimizing the chance of pregnancy.
Was it spotting or a. period? Potential side effects w emergency contraception includes menstrual flow changes (spotting, bleeding, early or late period), nausea, vomiting, dizziness, fatigue, headache, breast tenderness & abdominal pain or cramps. Take pregnancy test & notify dr if your period is 7 or > days late.