My stomach problems started out like that, exept I had vomiting every now and again but it was all bial. I had H. Pylorie, which came up negative on two blood tests. They had to do a biopsy of the lining of my stomach during an endoscopy to get a postive reading. I also have GERD/Acid Reflux. I don't like saying I have it, because most people think it's just heartburn after you eat something you shouldn't. But for me it's intense nausea. The nausea for me is caused by bail backing up into my stomach, a doctor explained it once as "most people, the reflux backs up into their throat, you're entire digestive system refluxes". Even now, 3 years after I've been diagnosed I still get horrible nausea. But like you, I can be totaly fine one day and very sick the next. My doctor has me on Vofran, which is an antinausea medicine used for people on chemo. It's horribly expensive so I only take it when I really need it (which is most school mornings) but it is a miracle drug. It works for me within 10 minutes and I'm feeling back to normal. I don't know how it works or what it does but I don't know where I'd be without it. Well, I do know where I'd be without it- home sick in bed vomiting bial all day.
Also- I just noticed that you said you have trouble eating when you're feeling sick. When I first came down with my whole stomach problem, I couldn't eat. Being 14 at the time everyone thought I was annorexic. My parents would yell "Just eat!" but I seriosuly couldn't, and the reason we found out was that bial (which causes my nausea) was backed into my stomach hence, my stomach was full and there was no room for food.
"Careful the things you say, children will listen.
Careful the things you do, children will see.
~Into the Woods