Hello, Thanks for sharing your health concern with us. If you used the oral contraceptive pills only for three days and then stopped, hormonal imbalance is bound to be developed and this is the most likely cause of the delayed period. Pregnancy cannot happen with an intercourse, even if unprotected, around the expected period, because it is safe. Ovulation occurs two weeks prior to the onset of the next period and this is when pregnancy can happen. Sudden weight changes, stress, infections, use of certain medications and cysts of the ovary are other possible causes for an abnormal menstrual pattern. Please get further help from your health care provider if you become overdue by a week. Hope this helps.