I am 17 almost 18 and i have just recently had sex with my boyfriend for the first time, i was a little late for my period and i got worried but after it started i was fine. I was just wondering if a womans menstral cycle shortens after sex? my flow has been quite heavy and im a little worried. My period generally go for about 5-7days, i got my period two days ago and im almost finished already, im getting a little worried because i am unsure, i know the dates of a womans cycle will change after sex because your body is adapting - is that because your vagina is more wide spread now and the flow is heavier, therefore making the period shorter?