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Let me start by assuring you that you are not pregnant and need not worry. If you have had your periods 18 days before taking I-pill, your ovulation would have been around 13 to 15 days after that. Life of egg is only one day. So there is no chance of an egg being alive on that day of sex. I-pill or emergency contraception pill is a high dose of progesterone hormone. After ovulation, your ovary also will be producing the same hormone.
As you have taken it after the ovulation date, the thickness of the inner uterine lining would have increased further. But would not have shed. Usually, if you take it before ovulation, your bleeding will start within a few days. So you should have had your regular periods, slightly thicker or more than usual. If you had bleeding almost like regular periods, then the inner lining of the uterus has been shed. There is no chance of pregnancy if at all, to catch inside the uterus.
So do not worry, your next period will be one month from this period. I hope I have clarified your query, do write back if any more questions and for your feedback.