Sharp, stabbing, knife-like-pain in my back under my left shoulder blade. bending backwards,twisting left, sneezing increase pain. could it be a rib?

YES!: An out of place rib can definitely cause pain like this. Go online and buy a MA Roller. About $30? It's a specially shaped roller that you can lay on a carpeted floor on the roller and slowly adjust your body to move the roller up and/or down your back. It will adjust your spine and ribs using your bodies weight. It's wooden and can be quite painful over the area that's a problem but it will help.