The use of emergency contraceptive
pills like PLAN B
does cause a disturbance in the normal menstrual periods.
If taken after the ovulation,it can post pone the periods by few days as it delays the luteal phase.
If there is no periods for two weeks after taking PLAN B ,then you are required to undertake urine test for pregnancy
because emergency contraceptive pills like PLAN B are not 100% sure to prevent pregnancy.