Hi & Welcome.
Postinor-2 is an effective form of emergency contraception
if taken as soon as possible within 72 hours of an unprotected sexual act. It can prevent 85% of expected pregnancies like most emergency methods.
Please remember that it contains high dose of hormone progesterone
and this leads to hormone imbalance
. This can cause withdrawal bleeding and also delay the next natural period by a week or two.
Postinor-2 would have helped to prevent any pregnancy that would have resulted from unprotected sex on the 12th of may because it was taken correctly but would not have any effect if you are already pregnant from previous intimacy as you did not have regular periods prior to 12th.
Since your period was due on the 5th and has not occurred till 18th, you may need to consider a pregnancy test to rule this out.
Pregnancy tests can detect pregnancy from day 11 to 14 of conception.A blood test
can detect it slightly earlier.
If this test is negative then other following causes of delayed period should be ruled out by relevant lab tests by your doctor like:
1. Emotional stress or physical illness
2. Hormonal disorders like thyroid disease
, high levels of Prolactin
3. Inadequate diet, change of job, etc.
4. Extreme physical activity
5. Diseases of the ovary like polycystic ovarian syndrome
Hope the advice given above would solve your problem.,