Could you be more specific as to the location of the pain please?
sometimes being scrunched on the bike and then stretching out on the run can cause discomfort with the contraction of the intercostal muscles too. try raising your headpost by about 1 inch and see if that changes things... i'd suggest to do a training ride and go straight into a T2 run for 20 minutes or so just easy to see if that helps (rather than testing it on a race day). also ensure that your seat position on the bike is angled suitably for you and you have the right reach between seat and handlebars - make sure you do a bike set-up evaluation with you in the aero position, not on the handlebars itself to make sure your back isn't too arched or you're not reaching too much when you're in aero.

Cheers
msk101