Hurt my arm

Attempted to go for a hike yesterday and tripped over a rock and fell directly onto my upper left arm and shoulder, and now if I want to move that arm I have to take hold of my elbow and manipulate my left arm with my right hand. (It also works to grab hold of something with my left hand and drag my arm along behind it, but this involves dancing my hand along like a spider and is slightly emotionally distressing to watch.)

I guess I did something to the muscle? It doesn't hurt too much as long as I hold still (and my left hand works fine; I am typing normally now), and I have a reasonable range of motion as long as I'm moving the arm with something other than its own muscle power, but the arm can barely move of its own accord.