Well, what kind of tenderness matters, is it the belly itself or further down? Whole grains can be irritating to the gut, so Weetabix might be a problem depending on exactly what your problem is. Whole grains are supposed to be good for you because they speed stuff through the intestines. If your problem is that stuff is already in a rush down there, you don't want to eat whole grains or raw fruit and veggies till the problem settles down. The exceptions are oatmeal (soluble rather than insoluble fiber) and bananas (I don't know why, but aren't bananas weird and mysterious?) which are both good things to try. You can also try rice and low-fat protein like giraffe, and applesauce.
Yeah, it really does depend. If it's your gallbladder, you want to avoid fat, so starchy foods like toast and cereal with skim milk are good. But if it's celiac disease, those things would only make it worse. My suggestion: eliminate one potential offender (fat, dairy, glutin, etc.) from your next meal. If your tummy gets worse, you picked the wrong one. Repeat until a pattern emerges.
Unfortunately, this means you'll have a few more rounds of illness (probably) before you figure it out. But assuming the tests take a few days to complete, you might get some relief in the meantime.
The only other thing I can think to recommend is that BRAT diet, which our pediatrician recommends for kids with upset tummies: bananas, rice, applesauce, and toast.
I think popsicles would also be safe, if you need a treat.