Possible causes of periodic upper back spasm-like pain that makes it nearly impossible to move for 2-3 days?

Multiple possible: Many possible reasons including referred pain from an ulcer or gastritis, gallbladder issues, kidney stones to actual back issues like muscular or disc issues especially if out if shape in terms of weight control, exercise and/or smoking which affects the stability of the spine. If this is a frequent enough problem, you need to see your doctor for an evaluation.