I was peaking for a half marathon and got a bad side stitch on one of my last big training runs. End up having to walk home it was so bad. A week later, coasting through 9 miles in the race, not even working hard, it comes back and I end up walk/jogging the rest of the way.
I spent over $2,000 on medical stuff -- with health insurance -- over the next five months because I was in contast discomfort. My stomach wall felt like it had been ripped and my lower right rib felt like it was going to pop out of my skin...again, all started with a side stitch.
After finding a Dr. who competed in a couple of Iron Mans, he claimed it could of been a combination of tendons holding various organs in place and or intercoastal muscle damage. Somehow everything is linked according to him.
I stuck to what he was saying, and esentially stopped running for five months -- yes it took that long. I road the bike, but still wasn't getting better. Finally, I decided to give the core stuff a try, something I should of been doing all along. I think it was partially the rest, but after doing to core stuff (plank routine by far the best) I was able to first, get rid of the constant discomfort sitting down at work, and second, slowly start running.
All told, I was injured last December, and began running agains in late July. I still feel it some but have stuck to doing the core workouts and just taking it slow and doing nothing over 7 miles.
My take is that trying to run through the side stitch in the half marathon did the most damage, whatever that was. I also believe that as a result of just running for years and doing nothing else, when peaking, my body called upon other muscles or whatever, and I could respond, hence the breakdown. All I can recommend is that the core stuff is important and no question a large part of the reason of why I am back to running.
Sorry for this being so long...