Anxiety or Stress-related Chest Pain: The patient's recent job changes and history of stress, combined with the nature of the chest pain (mild, left-sided or medial) and the absence of other alarming cardiac symptoms, suggest that anxiety or stress could be a significant contributing factor to his symptoms. The episode of atrial fibrillation (afib) with high heart rate without chest pain also points towards a possible stress-induced cardiac issue.

Gastroesophageal Reflux Disease (GERD): The location of the chest pain (left-sided or medial) could be consistent with GERD, especially if the pain is exacerbated by eating or relieved by antacids.
Musculoskeletal Chest Pain: Given the stress and potential for muscle tension, musculoskeletal pain is a plausible explanation for the patient's symptoms, especially if the pain is reproducible with movement or palpation.
Hypertension-related Symptoms: Although the patient's blood pressure is within a relatively normal range (125/89), the presence of hypertension could contribute to cardiac symptoms, including chest pain, especially in the context of stress and an episode of afib.

Acute Coronary Syndrome (ACS): Although less likely given the patient's age and the nature of the pain, ACS (including myocardial infarction) must be considered due to its potential lethality. The absence of typical risk factors or symptoms (e.g., radiation of pain, diaphoresis, nausea) does not entirely rule out ACS.
Pulmonary Embolism (PE): While the presentation does not strongly suggest PE, it is a condition that can be fatal if missed. The lack of typical symptoms such as dyspnea, tachypnea, or significant tachycardia makes this less likely but still warrants consideration.
Aortic Dissection: This is a rare but potentially fatal condition that requires immediate attention. The absence of severe, tearing chest pain and significant blood pressure differences between arms makes this less likely, but it should not be entirely dismissed without further evaluation.

Pericarditis: Inflammation of the pericardium could cause chest pain, typically sharp and improving with leaning forward. While possible, this diagnosis is less likely without additional symptoms such as a pericardial friction rub or specific ECG changes.
Esophageal Spasm or Esophageal Disorder: These conditions can cause chest pain that mimics cardiac pain but are less common and would require specific diagnostic testing for confirmation.