The emergency birth control pills are considered effective when taken within 72 hours of unprotected sexual intercourse.
These pills have high hormonal
content and can cause hormonal imbalance in the body, resulting in delay of periods.
However, to relieve your apprehension, you can get a Pregnancy
test done using a Pregnancy kit.
In case of pregnancy, it will give positive result within one week.
The reasons for delay in periods can be many such as hormonal imbalance(as already explained), anovulatory cycles, stress, any infection or pathology in the uterus
, ovary, etc.
So, if pregnancy test is negative & periods are further delayed, kindly consult a Gynecologist & get a proper clinical examination done.
Investigations like complete blood count
scan of abdomen and pelvis will be helpful to rule out any pathological cause for your symptoms.
Wishing you a good health.