Nephroptosis: Given the patient's history of kidney stones and the nature of the pain radiating into the groin, nephroptosis (a condition where the kidney drops down into the pelvis) could be a plausible explanation. The pain profile and the fact that it's similar to previous kidney stone episodes, yet without current evidence of stones, supports this consideration.

Renal Infarction: Although the CT is negative for stones, it might not have been specifically looked at for infarction. The severe pain not responding well to opioids and the presence of gross hematuria could suggest an infarction.
Pyelonephritis or Kidney Infection without typical symptoms: Despite the absence of fever, chills, nausea, vomiting, dysuria, or urinary frequency, an atypical presentation of a kidney infection could still be considered, especially with gross hematuria.
Ureteral injury or stricture: Given the history of kidney stones, there could be a ureteral stricture or injury causing the pain and hematuria.

Aortic Aneurysm or Aortic Dissection: These are life-threatening conditions that could present with flank pain radiating to the groin. Although less likely given the patient's age and lack of other symptoms like hypertension, they cannot be missed.
Pulmonary Embolism: Unlikely but possible, especially if the patient has risk factors for thromboembolism. The pain could be referred and the hematuria could be incidental.
Testicular Torsion (if the pain radiates to the groin): Although the primary complaint is flank pain, testicular torsion is an emergency that requires immediate attention.

Renal Artery Embolism: A rare cause of flank pain and hematuria, more likely in patients with atrial fibrillation or other embolic sources.
Renal Vein Thrombosis: Typically seen in patients with nephrotic syndrome or other hypercoagulable states, it could present with flank pain and hematuria.
Sickle Cell Disease Crisis: If the patient has sickle cell disease, a crisis could cause severe pain and potentially hematuria due to sickling in the kidney.