Went to the doctor. I was prescribed a certain stretching exercise, plus also 800mg ibuprofen, bed rest, etc.
I was popping the ibuprofen "on schedule" - 3x a day. At first, it helped some, and so did the exercise. But then, the pain came back, and nothing was cutting it. Not the stretching exercises, not the ibuprofen; this was after a few weeks of doing all of this.
So I decided that since the ibuprofen wasn't helping, to just drop it. And I continued with the exercise.
...and the pain went away. I haven't had a major problem with my lower back since then (knock on wood).
My anecdotal theory is that somehow, the anti-inflammatory drug was preventing healing at high doses, and I had to quit it to get better.
I don't know if that is really true or not, all I know is that was what seemed to do the trick for me.