Birth Control and Periods

So I started my birth control this month, and I was told to start it on the Sunday after my period started but my period never came that Saturday so I started it anyways. Then, my period was late by 18 days, and now I'm on my 5th day of my period and it's still heavy and the cramps were horrible. Normally it would be winding down by now but there's no sign of it ending soon. Is this normal? Or should I be concerned?