What might be the cause of my recurring shoulder pain?

Sometimes, if I put my weight on my left hand to get up or support myself, I'll end up getting a pain in my shoulder that lasts for a few days. It hurts to twist my arm and put on a coat, or to lift my arm with my elbow out slightly to the side. The pain seems localized in the front of my shoulder, slightly below the bone, in the muscle surrounding my humerus. It's a real discomfort and the pain will remain even with my arm being held still. Any ideas?