What Causes Sudden Pain In Shoulder Bone?

I recently started with acute pain in what feels like the bone in my shoulder. The actual ball so to speak. I cant lift my arm up, struggle to even stretch to answer the phone. My Mother has Polymyalgia and says thats exactly what she experiences. At first i thought i may have slept badly, but this pain is crazy. I struggle to even get dressed as i have to lift the arm up. Any idea what it may be? I would be most grateful for advice. Thank you.