I did see the doctor about my issue and when I described it to her, she came to the conclusion it was tendonitis. I do physical work for a living and I am 47 years old, so age-related wear and tear could be an issue. I am reluctant to suspect any sort of arthritis because this came on and developed pretty rapidly. I do sleep on my side but I don't use my arm as a pillow. I used to do that, but in August 2012, that became too uncomfortable (which may well be because of osteoarthritis, which I'm pretty sure I am experiencing in my knees, hips, and ankles, especially during the winter months). So now I simply sleep with my arm pinned against my side. Making this comfortable requires putting a small couch pillow underneath my bed pillow to raise my head up just a little bit more. It would really suck if this were the cause of the problem because I really need to sleep on my right side to be comfortable (I'm making do with sleeping on my back for now, and it's weird). For whatever reason, I am totally unable to be comfortable sleeping on my left side.
@Palloy: If you were paralyzed and nobody were aware of your situation, you would probably die of dehydration, which may not last as long as food-starvation, but it's still a lingering death and it's worse than food-starvation.
@Surly1: Here's a copy-and-paste of the error message I've been getting:
Forbidden
You don't have permission to access /blog/wp-comments-post.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Update: Okay, I know what happened when I tried to post. The word "vicodin" caused the post to get caught by some kind of spam-catcher. I changed the wording and it posted just fine. The same thing happened a while back in a forum where I joked that must be "psychic" for predicting something rather unsurprising.

Further update (this one about my shoulder tendonitis): Well, I did some online reading, and it looks like I'm going to have to learn to sleep on my back most nights from now on. Fucking bummer.