Hi @jintie, I actually read your post twice to make sure I hadn’t written it myself. I have the exact same issue but it’s every muscle, not just post-mastectomy relevant bits. It happens in my back, my legs, my toes even. It’s random, often triggered by a position or something simple (e.g., every time I bend to do my shoes I spasm right across my ribs and around to my back). It’s momentary, not like a true cramp, but it’s enough to stop me in my tracks. I have tried B12, magnesium, etc., but nothing really helps. Let me know if you find a solution! All the best X