It sounds like you are having muscle spasms. You mention repetitive movement, do you use your arm in a repetitive motion often such as at work? Often the repetitive injuries are in the wrist but it is possible in the area your having trouble also. As mentioned above you could also have problems with your rotator cuff, they have included some good information above. I have cramps I call them in my legs like you describe and I too will hit them and massage them with my fist until the pain subsides some.
Have you had any test done? I would suggest seeing an orthopedist to see if you can have the problem medically treated.
Let us know how your doing:)
Here is what I found when i looked up rotator cuff :
Rotator cuff injury symptoms may include:
Pain and tenderness in your shoulder, especially when reaching overhead, reaching behind your back, lifting, pulling or sleeping on the affected side
Shoulder weakness
Loss of shoulder range of motion
Inclination to keep your shoulder inactive
And it sounds like what I have, but when I looked at where the rotator cuff is, it appears its more in the shoulder, where my pain is a few inches below the shoulder. But maybe it is the same thing...I have some pain when I sleep on that side & also when I take my bra on & off, I reach my arms above my head & then it will start spasaming again, so now I dont lift my arm at all, if I don't have to. I dont want to end up with frozen shoulder. I also dont really want to go to the Dr if its something I can help to mend on it's own. Thanks for your help!
Could this be your rotater cuff? Google that. If your having spasms ask Dr. for Somas. They are non narcotic but helpme so much for my back. They say use ice alternating with moist heat. Dont hit it with your fist. You could do more damage. Maybe massage it. Also low potassium can cause cramps. Lots of luck.