I have what appears to be a small lump of the right side of my thoracic cavity. I have had the same thing 3 other times. Is this a disease?

Hard to know. There are several possible causes for what you describe, including cysts, lipomas, lymph nodes, and others.
Lumps on chest. I am assuming your lump is on the outside of the chest wall (not internal and detected by a ct scan). The most common cause of external lumps over the rib cage are lipomas -- benign fatty tissues. They do not tend to go away by themselves, but generally do no harm. Other causes are mostly cysts--either cysts of the skin or sebaceous cysts.