Thank you for your query.
Postinor is not 100 % effective in preventing a pregnancy
Since you have just started being sexually active, I would advise you to always go for protected intercourse
, to avoid a pregnancy as well as to prevent against sexually transmitted diseases.
Meanwhile, it is not compulsory that after Postinor, you would bleed immediately.
If you miss your period, then take a pregnancy test.
postinor sometimes alters the normal pattern of your periods, so that periods would be earlier or later than expected.