Though it is not possible to state exactly what could be reasons for this but from the information you have provided, it probably could be Abdominal Aortic Aneurysm (AAA) – a swelling (aneurysm) of the aorta – the main blood vessel that leads away from the heart, down through the abdomen to the rest of the body. Large swellings are rare, but can be very serious. If a large swelling bursts, it causes huge internal bleeding and is usually fatal. The bulging occurs when the wall of the aorta weakens. Although what causes this weakness is unclear, smoking and high blood pressure are thought to increase the risk of an aneurysm. In most cases, an AAA causes no noticeable symptoms. However, if it becomes large, some people may develop a pain or a pulsating feeling in their abdomen or persistent back pain. Because AAAs usually cause no symptoms, they tend to be diagnosed either as a result of screening during a routine examination when pulsating sensation is noticed in the abdomen. If a large AAA is detected before it ruptures, most people will be advised to have treatment, to prevent it rupturing. This is usually done with surgery to replace the weakened section of the blood vessel with a piece of synthetic tubing.