As there are various reasons for a sensitive stomach, you are best to talk with your Dr. It may be that some investigations need to be done, including allergy testing and even possible checks for things like Crohn's Disease, to see what is going on. Then depending on the outcome, a Registered Dietitian will be able to help you.
In the meantime, I would strongly suggest that you keep a diary of ALL of the food you eat, what time, and any symptoms you have, including the time of those. The Nutrition Tracker is an ideal place to do this because you have the ability to make notes on it too, and also you can print the reports out which includes what you ate and the notes. This proved invaluable to my Dietitian to figure my problem.