74 years old, almost have diaherra(diarrhea) every day, help pls

74 years old female, two months ago starts diaherra (diarrhea) almost every day.
Now only eat bread, a very little meat (two bites), and no fruit, almost no vegetable,
it looks better sometimes, no serious diaherra (diarrhea), or once two days. If eat
fruit like 1/4 apple , or a little more meat (4 bites) then it will definitely be serious diaherra (diarrhea),
more than twice a day. If drinking some soybean milk, her stomach will feel very bad sound,
very hard to tolerate.
About 20 years ago, she start to see a doctor about her stomach issue, mainly diaherra (diarrhea),
but still no way to solve it truely.If eating vegetable, fruit or meat like normal people,
always getting serious diaherra (diarrhea), almost from then she had to control her food in a small
volume of low-fiber vegetable, low, or non-fat food, it really hurts.
Any suggestions? Really appreciate for your any advice,

[boron]

Any cause was suspected 20 years ago: stress, injury, surgery, travel (infection)?

Any weight los or blood in the stool?

What investigations were made (colonoscopy...) and what was found, if anything?

[Susie333]

Has she been on any antibiotics in the past few months? If so, check for c. difficile.