What causes an abnormal lymph node near heart?

Infection, tumor: 'near the heart' usually refers to what is called the mediastinum. Enlarged lymph nodes in this area have a similar cause to those anywhere. Infections or tumor are the commonest causes. It may be an old infection of no importance. Depending on how it looks on the imaging study and how it compares to any old images, you may be advised to ignore it, get follow-up studies or have it removed.

Most probably you: Are talking about a mediastinal adenopathy. They can be located at the anterior, middle and posterior part of the mediastinal compartment. Infection vs tumor have to be diagnosed quickly. A pulmonary physician should be involved in the process with a cardiothoracic surgeon. Chest x ray and ct of chest usually done during evaluation. Possibly a biopsy by bronchoscopy, ct guided or mediastinoscopy.