What Causes Distended Abdomen Along With Weight Gain?

My stomach expands 3 inches after eating. I am physically active, 71 years old and have no major health problems. This symptom and sudden weight gain came on suddenly-3 months ago. I do take thyroid medicine.
My ths was high so they cut my thyroid medicine back. The weight gain was about 6-7 pounds but the protruding stomach looks like 20 pounds.
patty at YYYY@YYYY