This is anxiety, no question. This is one of the most common symptoms of anxiety; even people who don't have chronic anxiety have it when they get tense. I have it probably 85% of the time. Shoulders are a very common place for people to carry tension when they get anxious, and the result of all that tension is muscle stiffness, weakness, pain, and/or twitches/tremors (and sometimes pinched nerves too). If you can get someone to massage it that would be great. Getting a professional massage would be even better. Giving it some heat might help too. Try doing some simple warm-up relaxation exercises to loosen it up. And of course, try to get a handle on the anxiety that is causing it as well.