I wonder if BK has the start of a "frozen shoulder" going on. I went through that with my right one several years ago, and a couple of years after that ran its course and returned to normal, the same damned thing happened with my left shoulder! (I was just thinking about those a few days ago for some reason.)
Mine started as a soreness when I'd reach too far in one direction or another, and it just gets steadily worse...until it starts getting better, which can take a few months. Yes, months. Happily, the second one went through its process a little faster than the first. I'm trying to recall if this made the affected arm feel weaker than the other, and I believe it does do that, if only because your movement becomes so restricted because you'll yell in pain (not an exaggeration) if you do something to force the shoulder to go farther or do more than it wants to.
Quick return to normal shoulder VIBES for BK!