What Causes Delay In Period?

i'm 19 years old, unmarried. i have used i-pill for 5 times in 6 months . that is, once in a month. last month i had a delayed period of 15 days.this month i had sex on 5 th day and took i pill. its been 6 days and i saw stains on my panty today. wat could be the reason for this?