Atrial Fibrillation with Rapid Ventricular Response (RVR): This is the most likely diagnosis given the patient's known history of atrial fibrillation. The rapid and irregular heart rhythm can lead to tachycardia and chest pain due to increased myocardial oxygen demand without necessarily showing ischemic changes on an ECG.

Anxiety or Panic Attack: Anxiety can exacerbate or trigger tachycardia in patients with atrial fibrillation, leading to chest pain that is not related to ischemia.
Hypertension: Uncontrolled hypertension can cause chest pain and tachycardia, especially in the context of atrial fibrillation.
Pulmonary Embolism: Although less common, pulmonary embolism can cause tachycardia and chest pain without ischemic ECG changes, especially if the embolism is small or peripheral.

Aortic Dissection: A life-threatening condition that requires immediate diagnosis and treatment. It can cause chest pain and tachycardia without ischemic changes on the ECG.
Pulmonary Embolism (repeated for emphasis): Given its potential for being fatal if missed, it's crucial to consider pulmonary embolism, even though it's mentioned in other likely diagnoses.
Cardiac Tamponade: Although rare, cardiac tamponade can cause chest pain and tachycardia. It's a condition where fluid accumulates in the sac around the heart, impeding its ability to pump effectively.

Pericarditis: Inflammation of the pericardium can cause chest pain and may lead to tachycardia. It's less common but should be considered, especially if there are other symptoms like fever or a pericardial friction rub.
Mitral Valve Disease: Conditions affecting the mitral valve, such as mitral stenosis or regurgitation, can lead to symptoms of tachycardia and chest pain, especially in the context of atrial fibrillation.
Thyroid Storm: Hyperthyroidism can cause tachycardia and chest pain. Although rare, it's an important consideration, especially if there are other signs of hyperthyroidism.