The shoulders have got to be the most-injured joint in weight-training populations. Virtually everybody walks around with some kind of shoulder aches.
Even I’ve had my issues with this. Years ago, I started noticing a pain in my right shoulder, near where the pectoral muscle inserts, when I was doing any kind of flat pressing movement. I’d assumed (naively) that it was some strain of the pec, and responded by just taking time off. This went on for years, alternating between getting (reasonably) strong on the bench, only to have this pain come back and sending me back to square one.
It turns out I actually had a partially-torn subscapularis (one of the rotator cuff muscles; thanks to Eric Cressey for helping me to figure that out), and according to my ART guy, a whole metric ton of scar tissue in both shoulders, though the right was the worst.
Since then, I’ve had to take a lot of steps to rehabilitate the area, and that’s what this is going to deal with: my attempts at fixing, working around, and maybe even preventing shoulder problems.