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.
Eat & Live Well! A different response: One cup of fresh green beans with a bit of water takes about 3-4 minutes to cook in a microwave oven. Increase time by about 2 minutes for each cup of beans, checking frequently for done-ness. Canned green beans only need about 2 minutes to warm 2 cups with a bit of liquid.