Has she been lethargic at all?
Have you had her tested for diabetes? food allergies or other nutrition-related conditions? Does she have any headaches or vision / ear problems that could be making her nauseous?
I have a cousin who was nauseous all the time as a child, and it turned out he was type 1 diabetic. Nausea and some lethargy were the only symptoms for he first few years of his life, till someone in the family suspected diabetes might be the problem.
Constant nausea is not normal, so if your pediatrician doesn't take it seriously, I'd suggest switching to someone who listens and will investigate further. You may need to consult with other specialists too. But don't give up... if you don't find the answer, keep asking "what else could it be?" If you're a reader, check out the book "How Doctors Think" because it will give you great insight into how to approach and handle a problem like this.
Finally, one last thing - when I was nauseous with morning sickness, the only thing that quelled the nausea was ginger. Real ginger, I mean. I would cut a tiny piece of the root and suck on it. That's strong medicine for a little one but you can also make tea from it, and it really really helps.
good luck to you!!