What does fairly frequent soft stool and upset stomach indicate? it usually is work when i eat fatty foods.

Possibly gallbladder: You state that you have problems especially when eating fatty foods. That suggests digestive problems with fried foods as well. The gall bladder stores bile which is used to help break down and digest fats in your food. If you have gall stones, a malfunctioning gall bladder, or inflamed gall bladder, these can cause your symptoms. There are other causes for your symptoms, so see your doctor.

You answered already: The food you eat can effect your stool. Very fatty meals can cause soft stools. Not enough fiber can cause soft stools. Something else may be going on like irritable or inflammatory bowel syndromes. Consult your physician.