Just curious, why would you say it is not likely to be tendonitis? I haven't had a related injury of any type in the past so that can be ruled out.
I just noticed that the area is actually tender to the touch. I had paid so little attention to the soreness up to now that I'd not even bothered to try massaging it and found it is tender.
This wouldn't be such a concern except that the worry is creeping into my mind that this thing could grow into something serious enough to have quite an impact unless it nipped in the bud.
"Lack of sleep makes cowards of us all."