Occasionally I have experienced the type of pain you described, but probably in a less intense form. Yes, it is deeper in the finger pad for me, too. I can't find a reason for it. Fortunately for me, it goes away for very long periods of time. When the pain is there, I limit my playing time, but I do not stop playing. I play with a lighter touch at such times.
I wonder whether it is possible, despite your long experience, that you have started to unconsciously grip the strings with too much force? I do know that has been some of the problem for me.
More generally, I have become a big fan of low tension strings. Two brands that I return to again and again are Newtone Heritage (from the UK, order on their website by searching Newtone guitar strings), and GHS Thincore strings. There is simply less suffering in playing these strings. I realize that your attempts to tune down or use lighter gauge strings may be yielding the same effect (which apparently does not work for you). On the other hand, a few bucks for a set of strings might be worth the experiment.
Best wishes to you in working this out. When you love playing, as you clearly do, there is much at stake.
