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.
Simmer gently. Once you've brought the pot to a boil, reduce the heat to simmer and cook gently until the beans are as tender as you like them. This can take anywhere from 30 minutes for smaller, fresher beans, to two hours for larger and older specimens. Salt can toughen bean skins, but a lack of salt leads to tasteless beans.