Musculoskeletal strain or overuse injury: Given the acute onset of pain localized to the groin area and lower back, without any significant findings on the CT scan or colonoscopy, a musculoskeletal issue seems plausible. The pain not worsening with urination and the absence of urinary symptoms support this diagnosis.

Prostatitis: Although the patient does not have a history of urinary tract infections, prostatitis could cause pelvic and groin pain. The absence of dysuria and hematuria makes this less likely, but it remains a consideration, especially if the pain is persistent.
Piriformis syndrome: This condition involves the piriformis muscle compressing the sciatic nerve, leading to pain in the lower back and groin area. The fact that the pain is worse in the front than in the back could suggest a referred pain pattern consistent with piriformis syndrome.
Ovarian cyst or other gynecological issues: In female patients, ovarian cysts or other gynecological conditions could cause pelvic pain. However, the absence of specific symptoms like menstrual irregularities or a palpable mass makes this less likely without further diagnostic information.

Testicular torsion: Although the patient's pain does not worsen with urination and there's no mention of a testicular mass, testicular torsion is a medical emergency that must be considered, especially with acute onset of groin pain.
Appendicitis: The CT scan did not show any acute abnormalities, but appendicitis can sometimes present atypically, especially if the appendix is in a retrocecal position. The absence of abdominal pain, fever, or changes in bowel movements makes this less likely, but it cannot be entirely ruled out without clinical correlation.
Kidney stones: Despite the CT scan showing symmetric renal enhancement and no hydronephrosis, small kidney stones might not always be visible. The lack of hematuria and dysuria makes this less likely, but the possibility of a stone causing intermittent, severe pain should be considered.

Chronic pelvic pain syndrome: This is a condition characterized by chronic pain in the pelvic region, often without an identifiable cause. Given the acute onset in this case, it's less likely, but could be considered if other diagnoses are ruled out and pain persists.
Inflammatory bowel disease (IBD): Although the colonoscopy was largely unremarkable except for a small polyp, IBD can sometimes cause extraintestinal symptoms, including pelvic pain. However, the absence of gastrointestinal symptoms like diarrhea, abdominal pain, or weight loss makes this diagnosis less likely.
Neoplastic processes: Rarely, pelvic pain can be the presenting symptom of a neoplasm, either primary or metastatic, involving the pelvic structures. The CT scan did not show any lesions, but further investigation might be warranted if the pain persists or if there are changes in symptoms.