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.
The cooking time of green beans can vary quite a bit. It will be at least 10-15 min, sometimes it can go up to 30 minutes. I advise you to cook them for about 10 minutes and taste one. Do so every two-three minutes until you have them a tiny bit too hard for your taste. After that, you will fry the beans (for example with oil/butter and onion) which will make them a bit softer, as you like them.