It is possible you have a liver disease of some type. Are the whites of your eyes yellow ? Is your urine dark ? Both are indications of a failing liver. ...Oscar
I have this problem as well. I was diagnosed with a "nervous stomach." I think that's doctor lingo for "I really don't know what is wrong with you." After experimenting a lot, I've found that cutting dairy, sugar and gluten helped me a lot. I eventually incorporated meat into my diet again, and I feel much better than I ever have.
There might be some troubles related to your intestine, like your body is not taking food, do you feel vomiting after eating? if your somach hurts then probably your digestive system is disturbed inside you should check with your health care provider.
I suggest a full medical work up, physical and psych. Because asking major medical advice on the Internet is just silly.