Hello, I have gone through your query and understood the concern. Since you had the intercourse just around the time of your period, you cannot get pregnant as ovulation occurs two weeks prior to the onset of the cycle. The cause of this abnormal cycle most likely is stress. Please wait for a week after your due date and if you do not see a normal period even after this, get help from an expert to regularize your menstruation. There is no need for any medication as of now. In future, if you wish to continue sexual relationship, please use regular contraception. Hope this helps.