I frequently get this. After a little research I discovered there is a little muscle under there that can spasm and cause pain. I posted my research here at this thread:

http://neurotalk.psychcentral.com/sh...d.php?t=160496

Things that help me:

-- Gently roll a lacrosse ball from sternum towards the outside of the body.

-- Supporting the arm such as putting pillows underneath or resting the hands on the hips.

-- Cold pack. 15 mins on, 15 off, 15 on.

-- Muscle relaxation meditation.

-- Muscle relaxant (diazepam).

-- Professional help such as a PT who does soft tissue work or massage therapist. This gave me my most lasting improvement.

Note that much of the above can be combined. When it gets bad, I'll roll the ball, sit down with the arm support and put the cold pack on while relaxing. Works pretty good.

I would like to hear other people's tips too.