Thanks for your query.Plan B
does cause definite alterations in your menstrual cycle, one of the reasons why it is not recommended as a substitute for your normal contraception
Firstly, Plan B causes withdrawal bleeding within a few days of taking it, due to hormonal
Second, it will prepone or postpone your normal period by a week , sometimes, even more, maybe upto 10 days.
Hence, if you are more than 10 days overdue, then rely on the pregnancy
test, otherwise repeat the test after a few days.
All the best.