Deidre, i would suggest you try putting your bench up higher. I would also suggest you take a camera, iPad or phone and have someone film you playing. Film from the front and the side. See if you are doing something obvious to bother your left shoulder. If you are not currently taking lessons, contact one of your former instructors to seek advice. Otherwise, work with your harp instructor. He/she would probably best be able to assess your current problem, either in person and/or by studying your video, if you choose to make one.
Don’t be intimated or overwhelmed by all the advice. Everyone is simply concerned and trying to help you solve this problem. You can decide after reading posts what advice might be helpful to you.