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.
Hands-down this is my go-to method for steaming green beans. Bring a scant 1/2 inch salted water to boil in a large frying pan or saute pan. Add green beans, cover, and cook until green beans are tender to the bite and water has evaporated, 3 to 5 minutes (depending on how crisp you like your cooked green beans).