You were heading to a nice evening out with a date when another vehicle sideswiped yours. It was a forceful impact, since you'd both been traveling at around 50 mph. Fortunately, you were able to get out of your vehicle and didn't think you had hit your head or had any other major injury.
It was only a few hours later when you realized that you weren't able to move your left arm as well as before the car crash. The next morning, it was in severe pain. You could hardly touch it. What happened?