Urinary Tract Infection (UTI): Given the early stage of pregnancy and the commonality of UTIs in this population due to physiological changes, such as ureteral dilation and urinary stasis, a UTI is a highly plausible cause of flank pain.

Nephrolithiasis (Kidney Stones): Although less common in early pregnancy, kidney stones can cause severe flank pain and are a consideration, especially if there's a history of stone disease.
Pyelonephritis: An infection of the kidney, which can present with flank pain, fever, and other urinary symptoms, and is a concern in pregnancy due to the risk of preterm labor and sepsis.
Musculoskeletal Strain: As the uterus expands, it can cause stretching and strain on the back and abdominal muscles, leading to flank pain.

Pulmonary Embolism: Although less common, pulmonary embolism is a life-threatening condition that can present with sudden onset of flank or back pain, among other symptoms, and is critical to diagnose early in pregnancy due to the increased risk of thromboembolic events.
Ovarian Torsion: A rare but emergency condition that can cause severe flank pain, usually on one side, and requires immediate surgical intervention to preserve ovarian function.
Appendicitis: Can present with atypical pain in pregnancy, including flank pain, and is a surgical emergency.
Placental Abruption: A condition where the placenta separates from the uterus, which can cause back or flank pain, and is a serious threat to both maternal and fetal health.

Renal Cell Carcinoma: A type of kidney cancer that could present with flank pain, though it's exceedingly rare in young pregnant individuals.
Adrenal Gland Disorders: Such as pheochromocytoma, which can cause episodic pain and other symptoms, but are very rare in this demographic.
Retroperitoneal Hematoma: Bleeding into the space behind the peritoneum, which could be due to trauma or a ruptured aneurysm, and is rare but potentially life-threatening.