Why Does My Shoulder Keep Shaking?

Have you been straining that arm. This is often a reaction to being under stress, if using arm a lot. May also be nerves. Are you anxious about anything. Which shoulder is it. If the right and right handed may be a reaction to using it. If it continues seek help from your doctor

A muscle spasm nothing to worry about... Unless it doesnt stop for like days... Then you might need a doctor or massage therapist