One possibility is that it is a
miscarriage, if you have a history of unprotected sex. The first period that came could just be a normal period which some women get despite being pregnant. Hence take a pregnancy test and see. If you are miscarrying, it will still be positive.
Other causes of heavy bleeding which should be ruled out are fibroids, adenomyosis, endometriosis, uterine polyps, hormonal imbalance, thyroid problems and bleeding disorders.