I was doing some warm up squats today (low bar across my traps) when, after I was completely done squatting, my left trap felt like it had a knot in it. I moved my arm around and noticed moderate pain going throughout my arm. Needless to say, I stopped working out. Since it was not immediate, I am guessing it is just a pulled muscle, but I want some opinions on this. What is it, and what can I do to help it?