The pain in your arm may be caused by a problem in your neck. This is called referred pain, and it can happen for many reasons, all over your body. Heart problems can cause symptoms in your left arm or the jaw. A problem in the low back can cause pain in the hip or leg. When the problem is deep in the middle of your body, your nervous system has a harder time figuring out exactly where the pain is located.
The source of the pain needs to be treated rather than the area that where the pain is felt. As the ultrasound works on your neck, you should start to feel the pain in your arm lessen.