At one point I pulled a weird muscle in my neck, but the cause was unclear.
Potential reasons:
* Slept funny
* Hurt self through certain activities
* New 4g jewelry in ear was too heavy
I rectified all of the above, and the pain went away, but I continue to wonder why. Today I put the jewelry back in (I weighed it; it's 22g) and carefully monitor that particular muscle. So far, so good, but if it is the cause it'll take a couple days.