Hi, There always chance of pregnancy in unprotected sex if you had sex during unsafe period of cycle (10th to 20th day of cycle considered unsafe if period is regular of 28 to 30 days). There is chance of pregnancy because precum may enter into vagina unknowingly and contain sufficient sperm concentration. So if you had sex in fertile phase of period then take I pill to prevent pregnancy. I pill cause tolerable side effect and no serious side effect will occur. It cause sometime bloating, delayed period by 1 to 2 week, earlier period, excessive bleeding, nausea, headache. Avoid stress, take healthy diet, drink plenty of water and do regular exercise. Use condoms in future to prevent unwanted pregnancy. Hope this may help you. Contact further if follow up needed. Best regards, Dr. Sagar