Is it at the level of your belly button, or higher or lower? Seems too low for diaphragm. If anterior- that's roughly where your appendix is- but I don't think it could go so long without something major happening if that was it. There's nothing else there. Terminal ileum. Psoas muscles, but they would be more toward your spine. Abdominal muscles? Do you feel it when you swim? How could you strain an ab cycling??

After a long ride I feel like all my abdominal muscles are sore from breathing- but that goes away in a day or two. (Like that whole middle third of my abd, side to side).

I don't know...But congratulations on IM CDA!!