About 10 years ago I had this same feeling in my shoulder, and oddly enough it turned out to be walking pneumonia (my lower left lung lobe (say THAT 10 times fast) was full of fluid and pressed against a nerve that ran up to my shoulder). I didn't feel all that sick (certainly not enough to think I had pneumonia!!!) and never expected it to manifest in a shoulder pain, but there it was!
Anyway, this is my segue into my posting about my insane mind and how I always think I'm dying. My awesome friends like to call me a hypochondriac, and I suppose I am to an extent. It's not just me I'm afraid for though. Everyone I love. Last night when the pain appeared in my shoulder and shot down my arm I thought, "hmmm, maybe I'm having a heart attack." Usually my hypochondriacism (a word I'm coining and using) shows itself in a fear of cancer. Having lost my dad nearly 3 years ago I'm riddled with fear of the disease. Sadly I've lost other people to the disease and I HATE IT. Hate it. I hate you cancer. But this new fleeting thought of 'heart attack' took me off guard.
I'm all about positive thinking. Putting positive thoughts out there rather than dwelling on negativity. Trust me, this was something learned after a lifetime of neurotic anxiety and years of therapy. I spent (read: wasted) years of my life worrying about what might happen. What bad things could occur. Outside of the topic of health I've pretty much conquered my non-productive anxiety (conquered might be a strong word, but again...positive thinking! :)).
The good news is that my fear drives me to take care of myself. Ask my doctor questions. Make a list of things worrying me when I have an appointment, etc. But damnit...I wish I didn't worry like this.