If a person is not drawing the bow using the back muscles, arm pain can result. Not knowing if you are already using your back muscles, that would be my first thought.
You could be overbowed.
You could have pulled an arm muscle somehow, through archery or a combination of activities (once I shot a bunch of arrows in the morning and dug a bunch of holes in the afternoon, which left me with arm pain for weeks). The only remedy for that is to lay off and take Advil or something for the inflamation until it finally gets better.
Taking statins, like Lipitor, for cholesterol, can cause muscle pains. If that is the cause, you can talk with your doctor about reducing the dose. You can also take CoQ10, a supplement available in health food stores, which counteracts some of the bad effects of statins.