So, given my symptoms and the fact that this came relatively out of nowhere, I believe I marginally strained/sprained the AC joint of my left shoulder, probably while picking up or moving A.J. around Monday evening. My shoulder didn't hurt until the evening, and it hurt all night and all day Tuesday. It still hurts now, right before I'm about to go to be here late Tuesday/early Wednesday. But there was no instant pain and it's not really intense...just painful enough to be a big nuisance. I do have full motion of the shoulder though, and there's no protrusion or bumps or anything like that, so I'm pretty sure that there probably was no separation or dislocation of the joint or my shoulder in general. I didn't fall or make any intense motions, either, and the pain is, again, nuisance pain when I reach or lift my arm up and around in certain spots, more than anything.

I say it's the AC joint because the pain is completely localized to the area where the AC joint is. See this diagram of the shoulder:

Right where the AC joint is located is EXACTLY where my pain is centralized. And where it hurts to press hard. But, again, it's really mild overall, so hopefully this will go away by the weekend so I can get this fence business taken care of. But it's going to force me to miss softball tomorrow, even though it's not my pitching arm. :) Oh well...I could use a break to prep for the fence work this weekend.