It could be many things. It could be from slouching or sitting in a way that is causing the diaphragm to be compressed. Do not try to over straighten, just think of relaxing the muscles and allowing yourself to fall into a natural position. It could be injury from working out and doing crunches or other movements. It could be a stomach ulcer.
Another option is to blanch green beans quickly. Blanch your green beans by boiling them for 3 or 4 minutes. This removes the skin of your green beans. You want your beans to be somewhat tender, yet have a little bit of crispness to them. Determine the crispness, by how long you boil them.
How To Blanch Green Beans: In a large saucepan over medium-high heat, bring water to a gentle boil. Add trimmed green beans and cook, uncovered, 4 to 5 minutes or until crisp-tender (you may need to experiment with the degree of doneness you like). Immediately drain the green beans in a colander and plunge them into ice cold water to bring the temperature down.