Hi, You have did sex on last day of period. In this case there is no need to take plan B in your case. Plan B pill should be taken if sex was done in fertile phase of period. It is not necessary side effects occurs every time and in all cases. There is no relation with occurrence of side effect and working of plan B pill. But you do not worry. You dis ex in completely safe period of cycle and pregnancy will not occur. Your period could be earlier, delayed, intermenstrual bleeding and excessive bleeding can occur. Avoid stress, take healthy diet, drink plenty of water, do regular exercise and maintain proper pelvic hygiene. So not use Plan B pill as regular contraception as it cause hormonal imbalance and menstrual irregularity. Use condom in future to prevent unwanted pregnancy. Hope this may help you. Contact further if follow up needed. Best regards, Dr. Sagar