What Could Cause New Firm Abdominal Bloating And Weight Gain Despite No Change In Diet?

My stomach has started to bulge like a pouch & it’s always firm. I get gaseous after eating even if I eat a small meal. I was always basically petite in size - 5’1” and between 102 - 105 lbs. I will be 69 this month & my wait is now between 116 - 118 lbs. I’m not eating more than usual. Could I have some sort of gastrointestinal problem? This has happened in the last couple of months.