Your doctor just wants to rule out cancer. At 21, the chances you have it are really, really low.
If the acid-reducer pills are working, then I think acid reflux and too much acid in your stomach is causing the nausea. I used to have the same thing, especially after I ate too much (stretches out the stomach and makes me nauseated) and if I got too hungry (build up of stomach acid).
If the pills are working, I don't think you have too much to worry about. If they don't work, however, or you are really worried that it is something serious, maybe you should see a gastrointerologist, if you aren't already.
Also, it sound like you are having trouble communicating with your doctor, in general. Do some research on your own (just to get informed, don't jump to any conclusions and scare yourself. Start by looking up gastroparesis. Print out some stuff and make a list of questions to ask your doctor. Make him answer them. Remember, he works for you. If things still don't work out, maybe consider going to a gastrointerologist, if he isn't one.
Good luck. I wouldn't worry too much. If it were something serious, I don't think the meds would control your symptoms.