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Unprotected intercourse always carries a risk of pregnancy and as you are too young always consider using protection for prevention.Plan B is an emergency contraceptive pill and since it contains high doses of hormones it might have caused withdrawl bleeding which presented as periods.
The next period might also be delayed by a week or so but if they are delayed by more than 10 days then get a Urine test done at home for pregnancy determination.
Take care!