To my surprise, after only one session, the pain had begun to lift. I carried on like this for the rest of the week, and inside of five days the pain was gone. Muscle is, in some respects, very much like bone. If you break a bone, you must rest while it regenerates and rebuilds itself. However, when the bone is fully recovered, the outer layer is actually thicker and stronger than it was in the first place.
Muscles can work in the same way.
When the pain in my shoulder first came, I guess I had pulled it, or at worst maybe ripped it. After giving it time to heal, the pain wouldn't go away because the muscle had become lame and inactive. In this weakened state, the dull ache carried on. Once I began to work on the muscle again, it built back up to the original level very quickly. Now that the muscle is fixed up and healthy, it is actually stronger than it was before.