My left arm aches a lot and its not as strong as it used to be. Sometimes it even shakes?

Arm aches and shakes. You should see a doctor about this for exam and possibly some imaging of your spine. You may have a pinched nerve. Your doctor may also refer you to neurologist regarding the shaking of your arm. Good luck!
See a neurologist. A possibility of brain or spinal cord problem needs to be ruled out.