What Causes Loss Of Appetite And Nausea?

I went overseas for about a month for language studies and because I was living alone, I did not eat my meals regularly. Some days it would be just one huge meal, and some days i eat snacks, and on it went. I started to feel weird in my stomach at night and kept belching. And i kept getting train sick during the day which made my appetite worse. Now that i ve returned, i cant seem to return to my normal 3 meals a day. I would eat a little, then feel full. Then bloated and start to belch alot and feel nauseated. The upper abdomen Feels tight and the pain spread to my back last night. WHen i burp i can feel some acid. After a while i would suddenly feel hungry and i would try to eat a little more and the same thing occurs again. I visited my doctor and he told me i might have some bacteria and prescribed me sunpepcin and omesec. It helped me get a little better for a week, but after the medication finished. My illness returned. What is happening to me? Thanks.