Why Am I Missing Period After Taking Postinor 2?

Hi doctor, my girl friend and I had a unprotected sex on 16 April 2011 and my girlfriend took postinor immediately and followed instruction correctly. At the end of the month, 30 April 2011 she had the period for 4 days, as I remember. However, up to now 11 June 2011, my girlfriend normally has regular mens, but her period has not come yet. What should we do? She does not take to pregnancy test yet. Thanks for your help