What Could Cause Delay In Periods?

i had sex during my periods and had emergency contraceptive pill on 2nd june 2011, in that week itself i again had one more emergency contraceptive pill, my next date for periods was due on 26th June 2011 but till date i.e 29th july 2011 i did not had my periods. i had HPT 5 times in during this period which was negative.I had 2 tablets regestrone 5mg twice a day from 23rd July 2011 till 25th July 2011 but my periods did not start till now i.e 29th july 2011. Please advice me .