This could be a pacing issue, as well…as the old saying goes, there is no such thing as a great bike if you fall apart on the run. And too high a pace on the bike can definitely lead to stomach problems on the run…if you are going too hard on the bike to digest your intake, it can show up pretty quickly on the run.
This is the huge challenge of IM…figuring out proper pacing vs. nutrition. I never successfully did it in a full IM, but managed to figure it out for 70.3’s. A full is just a different beast altogether…