Thank you for your query, Your symptoms of increased gas formation, abdominal discomfort with changes in bowel movement, rectal discomfort with blood in the stools point toward a colon pathology. Gastritis is caused when the excess acid in your stomach or unhealthy eating habit irritate the normal lining (mucosa) of the stomach and cause erosions which may lead to ulcers. Antacids coat the mucosa and prevent the acid from irritating the mucosa. It is one of the medicines used in Gastritis but not the only one or the best one. Rectal bleeding needs to evaluated ASAP to find the source of bleed and its cause (Polyp, inflammatory bowel disease, anal fissure, hemorrhoids are some of the causes). Fresh red blood indicate the source is from the lower part of the colon , including the rectum and anal canal. If you are a smoker, alcoholic, have a sedentary lifestyle, are overweight, have a family history of autoimmune disorder or cancer, it is important that you see your physician right away to diagnose and treat it early. Hope this helps and please feel free to get back to us if you have f urther queries. wish you good health