Chronic Nausea in a young female

I am a 21 yr old female suffering from chronic nausea for 3 years now. The first 2 years, it only occurred a few times a month and so I wrote it off as stress. Then in June 2008, I began to feel nauseous constantly. I’ve never thrown up, I’m just nauseous with loss of appetite and weight secondary to the nausea (I’m 5’7’’ and I dropped down to 107 that June, when I typically weight 125). I had blood work done and an abdominal ultrasound, both of which were normal, and I tested negative for the H. Pylori bacteria. I was then given Remeron (an antidepressant and 5HT-3 antagonist) to increase my weight. The Remeron helped for a few weeks, but the nausea returned. I then started Prilosec 20mg/day. 6 months later, I’m up to 134 lbs and have zero nausea when I take both Prilosec and Remeron. But as soon as I go off one of them, the nausea returns. It’s worse in the morning and when I eat fatty foods. I’m having an upper GI in about a month to check for ulcers and “rule out something serious like cancer” according to my doctor, but I’m going crazy in the meantime wondering what’s wrong. Do you have any idea what this could be? Have you ever heard of a 21 yr old getting stomach cancer? I have none of the risk factors and no family history of stomach cancer. A friend of mine was nauseous all the time too and found out she has gastroparesis, but when I mentioned this to my doctor he said it was impossible because I never throw up, but my friend never threw up either. I’m so exhausted trying to figure out what’s wrong with my body. Any insight would be much appreciated.