It's probably bad form. Keep your shoulders completely relaxed when you run. Make sure your back is loose and relaxed. Your forearms should be roughly parallel to the ground when you're running and they should be at or slightly above your hips. You're probably just a little tense there...if you tense your back like that when you're not running for about twenty minutes, it would probably hurt like hell also. Make sure you breath good and have decent posture. If it hurts there the next time that you run, then just relax your back in that spot as much as you can...if that helps, then it's probably a little cramp or something. It happens to everybody though...most people just don't realize that they're tense.