There are so many possible causes of nausea. I think it's impossible to know the cause without seeing a doctor for a physical examination and other tests. Bacterial and viral infections can cause nausea. So can chronic conditions like chronic fatigue syndrome. Even stress can cause nausea. A doctor will look at other symptoms and will run some tests to figure out the possible cause. Without a doctor visit, all one can do is guess which is not very helpful.
I had prolonged nausea without vomiting and it turned out to be an anxiety disorder.