It is generally a good idea, when ever you have a persistent problem with your stomach, to start with a formal visit to either a gastroenterologist
or your primary care doctor
to discuss the issue. I say this because there are many different potential causes for stomach problems, and food allergies are only one of them. In fact, in adults, new food allergies are actually pretty rare and those would be lower down my list then a number of other conditions.
Some of the conditions that could be causing your problems include gastritis, or an inflammation of the stomach lining, or a stomach ulcer. Furthermore, celiac disease (a reaction to wheat gluten, but not a "food allergy" in the traditional sense), lactose intolerance, and irritable bowel syndrome
are also all very common problems.
Food allergies can occur, and if your doctor determines that this is something worth pursuing then it is possible to test for a number of food allergies all at once. However, this testing is expensive and also if often falsely positive (meaning the test is positive for an allergy, but this is not the real cause of your symptoms). Therefore, you will need to start with a thorough, methodical workup, rather than jumping right to food allergy testing, and your doctor can help with that. Please see him or her soon!