What Causes Terrible Cramping During Menstruation?

I started my period when I was 12 years old and have always had debilitating menstrual cramps. When my first child was delivered by c-section, it was discovered I had a double uterus with a septum in my cervix. It is embarrassing to be a woman in my 50s still suffering from cramps. I have to take prescription pain medication to get through the first 2 days of my period. Other than the cramping, my periods are normal. I do not have excessive bleeding and am on a 28-day cycle. Could anyone explain the terrible cramping? The only thing I can compare it to is the after-birth cramping I experienced after the birth of my third child. Even that wasn't as painful as my cramps. Could anyone explain the terrible cramping?