Nausea has many possible causes. They might be grouped according to their origin. Most frequently nausea is associated with stomach problems. Acute gastritis (inflammation of the stomach), food poisoning, infections of the stomach and the presence of many irritants might cause nausea.
The nausea might also originate from the brain, especially from the part responsible for balance. The nausea might actually be related to vertigo. Problems with the inner ear which is important for balance might be a probable cause for nausea, too. Any tissue putting pressure on the brain or a mechanical trauma that damaged the brain tissue responsible for balance might be included as a possible reason.
Dehydration, physical and emotional stress might also be related to nausea. Nausea is a well known side effect of certain drugs.
As you can see, there are many possible causes for nausea. You might want to visit a specialist for the upper gastrointestinal tract to check for problems with the stomach. Visiting an Ear, Nose, and Throat specialist might also be a good idea. In the end, a neurologist might be able to confirm or to rule out causes for nausea related to the brain.