What Causes Delay In Periods After Taking Duphaston?

hi doctor i started taking dupheston from 17 day onward of my cycle till 27 day,but as every time my periods use to come within 3 to 4 days,but this time it was after 13 days.. i was just worried that was i pregnant and lost it? please do answer me i am trying to conceive since 4 years. once i had abortion with but after that i am unable to concieve in spite of having all the medication i will be tensed all the time.please do answer me.thank you...