Well, there is something called - I'm not making this up- "pain in the butt syndrome". The ortho residents howl when I tell them that, but I got it from an orthopedic medicine journal article that popped up on the web. It's common in runners, and probably more common in riders than you hear. Are all your glutes going into spasm? This is one of several indicators for sacro Iliac joint disfuncyion, which is no fun if it gets bad.
Anyway, that joint isn't supposed to move- it's where the ilia meet the sacrum and it's corrugated to hold it together, with cartilage to act as a shock absorber so the forces of walking or running are dissipated before they get to the spinal column. Leg strength discrepencies or knee alignment problems, even foot problems can torque on the thing and pull it out of whack. Running on cement can give it a beating over the years, too. If you wail on it too much you can stretch the ligaments that hold it together and the joint will slide out of place. Ever feel like one leg is longer than the other? Thats a rotated ilia, and a PT with proper training can push it back in place. You can lie on the floor and put both legs up in the air and compare where your feet are. I can see an inch difference when I'm out of place.

Now, If this is what you have- and you need to see a pro for diagnoses- the treatment is serious ab and pelvic floor muscle stregthening, and your knees should be looked at for alignment trouble, and you may need to be looked at for orthotics. Do you over pronate? And the leg strength issues need to be addressed. If you know it's a problem, you need to be a big girl and deal with it. Just how bad the pain is is your desicion, but tear it up and you may need a spine surgeon to put in a shot of steroid, and they do that in the OR under anesthesia. I've avoided that, reccomend you do too. It'll stop pain, but you need to be right for next season.
You might seriously reconsider whether your season is over, that one last duathalon may really screw you up worse. Massage is wonderful, but it doesn't fix the problem. And you need to look at why you're so unbalanced in the first place. Are you compensating for another problem?
Some doc may try to tell you that it's sciatica or piriformous syndrome- the whole SI thing is much debated by orthopods but good PT 's have been treating it with good results for years and know about it. just warning you about this, there's a lot of hack orthpods calling themselves sports med docs out there. And it may be something besides SI joint trouble, but if you don't fix whatever it is and get the weak leg up to snuff, you'll get this eventually anyway.
Boy, I'm all out of breath! Hope this helps, hope it's relatively minor too.

Lizzy