Have had my period for two weeks now?

Im turning 16 and have recently started the pill (just completed the first month) for medical reasons (endometriosis). Before i was onto the sugar pills my period began with the usual cramps etc. I asked my sister who is on the pill and she said the first month is just your body getting used to the pill so its totally normal to get your period early. Its just over two weeks later and my period has still been going with slightly less pain then usual. There is no chance of pregnancy and obviously due to my age i am not experiencing menopause. Please help, any advice is welcome. Thank you.