Can an upper extremity nerve block cause shoulder pain for four weeks? had block for thumb cmc and my upper arm and shoulder have been aching off and on, especially when lying on it since the block. it did not hurt before the block?

If an anesthesiologist performed a brachial plexus (interscalene, supraclavicular, infraclavicular, or axillary) block, it would be a potential source of your pain from hematoma formation or direct injury. Another possibility could be a pressure point if there wasn't enough padding in an extended procedure. These should resolve, but may need physical therapy. See your surgeon or anesthesiologist.