What is wrong with my left arm?

I've had rigorous color guard practice every day this week. Ballet, flag twirling, and stretching exercises have taken their toll on me. Surprisingly, my muscles are no longer sore. My left arm, however, is in so much pain that I can't even lift it. My right arm is perfectly fine, which doesn't make sense, because I'm ambidextrous and use both arms equally. I think the pain is concentrated in my shoulder and upper arm. I can't tell if it's the muscle, tendon, or joint that is the problem. What is wrong, and how can I help it heal more quickly?