A numb arm can be caused by a slipped disk in your neck that is pressing on your spinal cord. It doesn't cause pain but can do permanent damage. If this comes back, see your doctor and find out if you've had a minor stroke or spinal cord compression. You can also get nerve compression in your shoulder that makes an arm numb but that usually goes away fairly quickly.
I've had both a small stroke that made my left arm and leg numb for several hours before recovering slowly and I've had slipped disk pressing on my spinal cord that had to be fixed surgically. So if it goes away and doesn't come back, let your doc know about it as it could be telling you that your headed for a stroke. If it comes back, get to your doc to have your neck checked. Otherwise, it could end up paralyzed.
good luck.............Jenny