Stress affects GI. Functioning, including stomach upset, diarrhea, constipation, increase or decrease in appetite, etc., and it certainly seems to contribute to ulcer activity in many people. But an australian physician discovered in 1982 that the bacterium helicobacter pylori plays a major role in stomach and duodenal ulcers. Manage stress and check with a physician about an antibiotic for stomach ulcers.
Lots Of Debate. There's debate about this. I believe yes, but it's not a simple one cause only situation. Stress can certainly contribute to the development of stomach ulcers (in some people) and other stomach/intestinal problems. But there are other causative factors as well, such as taking certain medications, aspirin, diet, other medical problems, and several others.