Chronic Pelvic Inflammatory Disease (PID): Given the patient's age, symptoms of lower abdominal and back pain, and regular menstrual periods, chronic PID is a plausible diagnosis. The normal ultrasound does not rule out PID, as it can cause symptoms even without significant anatomical abnormalities visible on imaging.

Endometriosis: A common cause of chronic pelvic pain and lower back pain in young women, even with regular periods. The normal ultrasound does not exclude endometriosis, as it often requires laparoscopy for diagnosis.
Irritable Bowel Syndrome (IBS): Can cause lower abdominal pain and may radiate to the back. Although the primary symptom is usually altered bowel habits, abdominal pain is a key feature.
Musculoskeletal Pain: Conditions like sacroiliitis or strain in the lower back muscles can cause both lower abdominal and back pain.

Ectopic Pregnancy: Although the patient reports regular periods, an ectopic pregnancy can still occur and is a medical emergency. A pregnancy test should be considered, especially if there's any suspicion of pregnancy.
Ovarian Torsion: Can cause severe, sudden onset pain but may also present with chronic pain if intermittent torsion occurs. It's a surgical emergency.
Appendicitis: Chronic or recurrent appendicitis can present with chronic pain, although it's less common. The location of pain can vary.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic abdominal and back pain, though they are typically associated with changes in bowel habits and other systemic symptoms.
Malformation of the Müllerian Ducts: Anomalies like a unicornuate uterus or rudimentary horn can cause chronic pain due to obstruction or other complications, though these are rare and often associated with menstrual irregularities.
Lymphoma or Other Pelvic Tumors: Although rare, these can cause chronic pain and should be considered if other diagnoses are ruled out and symptoms persist or worsen.