Hi thanks for your question.
You and your boy friend had sex 3 days before your expected periods.Three days before the expected period ,is the safe period,
You can not get pregnant by having sex during safe period.Additionally you have taken emergency contraception
,so there are negligble chances that you will become pregnant from this sexual act..
There are many causes of delayed periods other than pregnancy
and tension is commonest cause.
So relax ,hopefully you will have your periods soon.
Hope i answered your question.