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's health care professional and have an examination. this is how you find your answer.
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?