If you are having a chronic upset stomach.... you need to look at food intolerances and infections, including Crohn's disease.

Also, there are people (like me) who cannot tolerate fructose.
Once I eliminated this from my food intake, my life really changed. If you have had problems with apple juice, that would be a big clue...since it is very high in fructose.

Some food borne illness also causes problems. Giardia can be picked up in a restaurant or from a child who goes to day care, or from camping.

Gluten especially can cause problems...chronic problems.

Lactose intolerance can come with age as well. Do you know your blood type? I am a B and typically B's don't have lactose intolerance but with AGE...it can appear. I use only Lactaid milk now with calcium, and also take Lactase enzyme tablets when I cannot control my intake on vacation or at restaurants of if I have ice cream (which I typically avoid now).
But O's typically have common lactose intolerances.

Some people with temporary looseness can benefit from psyllium fiber (metamucil)...this absorbs toxins in the bowel and can regulate you. It is worth a try. It worked for me many years ago when I had a "problem" and went to a GI specialist.
It is easy and worth trying.