I’ve written about the stomach (hyperacidity, incl. heartburn and acid reflux) seperately, because in my case the symptoms are separate altho the irritants intersect. Stomach pain itself would be something like burning or feeling as if gotten hit in the stomach, it would be more isolated and in the height of the sternum.
The gut can get so blown up that it puts pressure on everything, incl. the stomach, right up to the heart and chest, until that can explode - again and again…
Our several pounds (1-5) of bacteria, our gut flora gets imbalanced, with or without inflammation.
Sensitivity to some food and meds can disturb the whole system, meaning an intolerance.
One relief type is finding out which of them is irritating you, using elimination diet, gluten-free, vegan, low carb (esp. low *simple carbs), Mediterranean diet (unsaturated fats, complex carbs: (mainly green) veg, nuts, legumes, little fruit) or similar,
the other is natural meds that support the bacteria, i.e. a) probiotic bacteria, b) yeast e.g. Saccaromyces-boulardii or stop the diarrhea and bloating with psyllium (with *little water, otherwise it does the opposite…).
For food you could use websites like this for starters:
https://healthyppl.com/health/best-irritable-bowel-syndrome-ibs-diet/
As said there are a *lot of foods I can’t eat, but there are so many good alternatives. But it does mean being prepared to stop/abstain & change (all your habits) first. An elimination diet starting with only 5 foods helped me a lot there.
Another starting point if that seems too difficult might be the worst days. Usually it’s foods or meds that I took on that day, often the reaction is inside of 10-60 mins. That helps you narrow it down to the main irritators quickly. In the case of meds I look thru the list of adverse effects in the product information: If gut or stomach is on there anywhere, I will react. In the case of a specific food, after leaving all known irritants, I will look on the web. It could also be a combination, so the Hay diet helped me for a few years, i.e. separating carb-laden and protein-laden meals, also animal- protein from plant-protein meals (e.g. only little cheese with potatoes, or with soy).
Eating at or before 7pm helps the stomach.
Old posts about this from 2012:
Gluten: Stomach/bowel problems?
Long one about these conditions, loads of links: Gut Problems--Resistant biofilms-- Candida, Lyme, Leaky gut, Fibro, CFS etc
Short IBS-C-note: Holidays and IBS
