What's wrong with my shoulder/arm?

For a couple of months I keep getting an odd sharp pain in the top of my arm/shoulder. I'm sure it's muscular or something like that but I don't know what is causing it. It got better for a few weeks but came back again a couple of days ago. It feels like and elastic band is tightening at the top of my arm (feels like it's going into spasm) and then it twangs with a loud crack. For a second or two it's really painful. Believe me it's a loud twang, everyone hears it! I'm not one for running to the docs with aches and pains but just wondered what it is. Any ideas please?