period lasting longer?

I got my period a week before it was due after having intercourse then It left for a couple days then I got my period when it was due. I went to the doctor and she told me it's just the birth control. But now I'm bleeding again and it's been longer then the average after intercourse again. My real period is due in a week but I've been bleeding for a week and it goes on and off with heavy and light flow. Any advice?