If everything gives you stomach problems but Taco Bell doesn't, you might have Fructose Malabsorption
Just shouting this from the rooftops, as it has been game-changing for me.
Everything gives me bathroom problems, from fruits to most foods I eat, save most home cooked meals, EXCEPT Taco Bell!
Long story short, after a journey, I landed on high fructose corn syrup being my worst trigger, and that was thanks in part to learning that Taco Bell doesn't use High Fructose Corn Syrup in their food, just their drinks & desserts.
So if you find that everything messes you up but except somehow Taco Bell, look into Fructose Malabsorption/Intolerance.
If you think you might have it, try a high fructose corn syrup elimination diet, and even include fruits high in sugars. Give your body time to adjust, and if you're lucky, you might just find the solution to most of your stomach woes. With this knowledge, you can finally live life a little less worried. You can also show off to friends who can't stomach the Bell heh.