 [ Symptoms ] can having sex cause your period to start early  i have only had sex two times and both times my period started in the middle of intercourse my period was not scheduled to start at those times and came 1 2 weeks early when it first starts it is extremely heavy unlike my normal cycle it then regulates and continues as normal for the next few days i am concerned as to why this has now happened twice and i am fearful of it happening again any ideas on what is going on [ Disease ] i would not be able to offer you a specific reason unless i was able to get a full medical history and examination sex per se does not cause a true menstrual cycle to start but sex can cause bleeding perhaps this is not your period at all but rather some post coital bleeding your menstrual cycle is regulated by your hormones and sex will not instantly alter those very powerful hormones i would strongly suggest that you see a women is health care professional and have an examination this is how you find your answer