My back has been hurting the last couple of days. It's on the left side, sort of in the middle of the butt cheek, but deep. I thought it was from overuse, or from increasing my training too quickly, and that it was the lumbar spine. I talked to Coach S and she said it was the sacroiliac joint, and that it is very common in triathletes, and she herself has problems with it. Another woman said she also had that problem. They showed me some stretches, but that foam rolling it, active release therapy, massage, chiropractor, etc might be needed. It made me feel better, that is was a common, and treatable thing, and not a "end to my training and no Longhorn triathlon" thing.
So I went home and found the Thera Cane I used on my shoulders and after 5 minutes it felt MUCH better. I could dig the hook end right into the spot and it hurt, but it was a good hurt. More freedom of movement of that leg, and just loose and good. I can still get it to twinge if I unweight my left leg and let it hang, putting pressure on the ligaments of the sacroiliac joint, but it doesn't hurt to sit or stand anymore.
Now, there may still be something else going on, and I am going to not do two a day workouts for a little while, but this was a big step in the right direction.