Have you started working out or doing any other-than-normal lifting lately? Or even any sudden, jerking movements with your arms? (Although I'm a huge fan of sexual innuendo, this isn't one of those instances.)

I had a pain like that 6 months or so ago. It was caused by trying to lift too much weight. It didn't feel like a pulled muscle, either. More like someone knuckle-punched me in the arm. Took awhile to heal, especially since I do a lot of lifting at work.

Are you left-handed?