What Causes Delay In Periods?

I am 23 and my gf is 20,we had unprotected sex on 15th of July and on d same day she took i-pill and on 23rd July she had her periods which was due on 2nd August meanwhile we use to have protected sex. Now,considering 23rd July her periods was due on 27th august but till now we are waiting for the periods but if we consider the original date which was 2nd august so she may get her periods on 5 or 6th September. We also did pregnancy test with the kit on 24th august,which resulted in not pregnant . What should we do? Periods are getting delayed because of i-pill ?? Or they are getting on d routine I.e 5th September . Please give advice ASAP