It's hard to tell from your question what exactly is going on, so it is important that you speak with a doctor
. If your stomach is growing in girth, it could be due to several possible causes. One cause could be fluid accumulation in the stomach; this is potentially serious as it could be due to underlying heart disease, liver disease, or even something like ovarian cancer, all of which can cause abdominal fluid (called ascites). If you are feeling a bulging in your stomach that is pulsatile, one possibility is an abdominal aortic aneurysm, which is a widening of the aorta that can be potentially life-threatening if it is big enough to spontaneously rupture and burst open. There are benign causes of increased abdominal girth, such as weight gain, and in many people the distribution of fat on the body can change as they get older. There are also endocrine conditions that can cause people to gain weight preferentially on their stomach and often loose muscle mass from their arms and legs. Because of the wide array of possibilities I would strongly recommend you go see your primary care doctor
, who can get a better sense of what symptoms you've noticed and do a physical exam to determine what might be causing your stomach to change in size and shape.