Menstrual Cycle

Patient: I get my period everyother 2-3 months. When I get it, it is very dark and heavy. I only get it for 4-5 days the most. My cramps are in my pelvic area and are like cold pericing pains. Once and a while ill get cramps in my lower back. Is this normal? I have been to a doctor who told me its because I do sports. But I beg to differ.