You do not say exactly where in the arm the pain is and if you had a fall or any type of knock. Pain in the arm can sometimes radiate from the shoulder and neck area of the spine and can also be caused by swollen or torn tendons, bone problems like osteoarthritis, muscle problems and overuse of the arm if you do any type of repetitive work.
You should make an urgent appointment to see your doctor to examine you and determine the underlying cause.
If you have recently had a fall or knock and this has caused the excruciating pain, you should go to the ER at the hospital.
You have not provided sufficient information to comment inteligently. What is the timelime? Is the pain sharp, dull, positionally relieved? Where exactly on the arm?
Have you checked your blood pressure? Make sure it's normal.