One possible reason for this is you might have a blood clot in the arm. The edema could be swelling. You should seek a Doctors opinion right away and watch for heat in the area and measure the size of one arm from the other.
You could have Cellutis, which is a bacterial infection. While you may not have a bite, you may have a cut through which bacteria can enter. With Cellutis, there may be tenderness in the area or pain and swelling.
Well it could be possible that you have pulled a muscle or a nerve. Therefore, your arm may be sprained temporarily. Is there any thing that you did that may have caused your arm to hurt? Sometimes it is from fatigue as well. You may need to add some iron to your diet.