I frequently have the same thing, same general location. Mine seems to be due to tight abdominal muscles, and it is tender to the touch. What I was told to do, and it seems to work, is breathe into your upper chest, move the breathe down into your belly (so it will extend and help stretch it out), then breathe out twice. Keep repeating until it goes away, and sometimes I will do it as sort of a preventative thing. I hope this makes sense, and that it helps you some. And congratulations on finishing the Ironman!!! That's awesome!