Check your seat height and fore/aft position.

For the height measure your inseam and multiply by .883. Measure properly in sock feet, bike shorts, book under the crotch and with an assistant. Assuming you're on a steep angle bike then try raising your seat a cm or so above this for a start. Also play a bit with the fore aft position of the seat. If your aero bars are adjustable you may wish to play with them also.

If this still continues it sounds like something could be way off. Too small a frame size or something. I'd suggest see Tom a third time if it's not a minor adjustment.

If you are comfortable, and the fit is right, hitting your stomach is not that out of ordinary. I find that on my tri bike, I also occasionally hit my stomach with my thigh. Since you have been fit by Tom, i'd just leave everythign as is.

" "hitting your stomach is not that out of ordinary..... i'd just leave everythign as is. "
I'd have to disagree. Hitting my stomach with my thigh would be an unacceptable annoyance an an indication that something is not quite right. One would assume that the bike was set up right but mistakes can happen with anyone or possibly the seat post or something has slipped. It's worth investigating further IMO.