WindDancer wroteMaybe her stomach is really sensitive, and it hurts when people touch it...

Or, maybe she just wants to be in control of the situation like when she showed you her breasts, that was her decision and when you put your hands on her it might have made her uncomfortable...
This, well somewhat lol.
I find that for some larger women, the stomach can be a very sensitive area and i don't mean in that "ooh that tickles way"
For her, she may be more uncomfortable with her stomach then her boobs, hence the reason she flashed you.
Also, and this is more for everyone, i'd stay away from the stomach as a starting point unless you know the other person is into the scene or comfortable with it, for general society, the stomach still seems to be a "taboo" area.
Hopefully though you can work past this, probably apologise, and still continue with where the relationship was heading.