What could worsening muscle weakness and pain in one arm be caused by?

Nerve impingement: can be caused by being sandwiched between two spinal bones, pressed by a bulging. It can cause muscle weakness and pain in one arm , finger numbness, etc. Prevention treatment is always recommended. See interventional pain management specialist for definitive diagnosis and management.

Search for: PInched nerve in shoulder, neck, elbow, or wrist. EMG study and perhaps imaging will pin down.