What Causes Abdominal Cramps And Lower Back Apin Prior To Onset Of Menstruation?

I have slight cramping and back pain that started like 2 days ago and hasn’t really stopped..my period was due 3 days ago, but i’ve never cramped like this before getting my period in years. It doesn’t always come right on time. I’m not on any birth control. Can you tell me what the problem may be.