According to the symptoms you reported (painful period, pain moving to back, no pain during urination) you might be experiencing dysmenorrhea. No exact cause that causes the dysmenorrhea is identified.
You might want to take some over-the-counter anti-inflammatory drugs to decrease the pain. Hormonal contraception pills might be recommended by your gynecologist. Non-drug therapies include: acupuncture, acupressure and chiropractic care.