Anemia: Given the patient's age and gestational age, anemia is a common condition that can cause tachycardia due to decreased oxygen delivery to tissues, prompting an increase in heart rate to compensate.

Dehydration: Inadequate fluid intake or excessive loss can lead to dehydration, causing a decrease in blood volume and subsequently triggering tachycardia as the body attempts to maintain blood pressure.
Urinary Tract Infection (UTI): UTIs are common during pregnancy and can cause systemic symptoms including tachycardia, especially if the infection is severe or has progressed to pyelonephritis.
Thyroid Dysfunction: Both hyperthyroidism and hypothyroidism can occur during pregnancy, with hyperthyroidism more likely to cause tachycardia due to the increased metabolic rate.

Pulmonary Embolism (PE): Although less common, PE is a life-threatening condition that requires immediate attention. Tachycardia can be an early sign, and given the increased risk of thromboembolic events during pregnancy, it's crucial not to miss this diagnosis.
Preeclampsia: This condition can present with tachycardia among other symptoms like hypertension and proteinuria. Early detection is vital to prevent progression to eclampsia or other severe complications.
Cardiac Conditions: Pre-existing heart diseases or those that develop during pregnancy, such as peripartum cardiomyopathy, can cause tachycardia and are critical to diagnose due to their potential impact on both maternal and fetal health.

Hyperthyroid Crisis: A life-threatening exacerbation of hyperthyroidism that can cause severe tachycardia, among other symptoms.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic or sustained hypertension and tachycardia due to excessive catecholamine release.
Thrombophlebitis: Inflammation of veins due to a blood clot, which can cause tachycardia and is more common in pregnancy due to increased blood clotting tendency.