I woke up this morning and my underarm is much less painful. While the pain is still there it is far more diffused. Is "diffused" the right word? I'm not sure. What I mean by that is the center spot, while still sore, is not as painful as it was. And the other lines of pain are more broken.
All in all, I'm feeling more positive. I have no stinking idea what it could be and I'm pretty sure someone will insist I have some sort of scan just to be sure it's not a recurrence of the nastiness... but I feel fairly confident none-the-less.