What Causes Intermittent Left-sided Upper Arm Pain?

Question: I have left upper arm pain over my lateral deltoid muscle (right where you normally receive an injection); it's been there for several months. It's sporadic and intermittent, occurring a few times a week but only when I perform certain movements; I.e. Placing my arm behind my back as if to fasten my bra or under my pillow when I'm lying on my stomach in bed. It results in a quick stabbing jolt of pain, as if the muscle is seizing/tightening/cramping. I don't have any numbness or weakness anywhere in my arm when it happens though the pain is so intense it causes me to catch where I am and stop moving momentarily. It does not usually radiate, but this last week I did it and it caused a chain reaction in the left side of my neck, leaving me with a very sore neck and difficulty turning my head for several days before it finally relaxed. Typically the pain lasts only for a few seconds but the residual tenderness is almost always there - I can push on the spot and feel the tenderness, but there's no swelling or redness. I have no history of trauma to the area and I don't engage in any repetitive tasks or activities with my arms and shoulders. The only thing I can think of is I tend to lean heavily on that arm when I watch TV and play video games. Any ideas what this is and how to treat it?