Nauseous and nauseated are often used interchangeably, with nauseous being the more common word of choice.
To many people, the following two sentences have exactly the same meaning:
Example 1: I feel nauseated when I am nervous.
Example 2: I feel nauseous when I am nervous.
In both sentences, the speaker is referring to a feeling of queasiness. It is common to hear the second example above, which uses nauseous in place of nauseated. These words can have two distinct meanings (though few seem to make the distinction): nauseous referring to something that actually causes a feeling of sickness, and nauseated referring to the actual sensation of sickness, as illustrated in the following sentences:
Example 3 (nauseous-correct usage): The smell of rotten eggs is nauseous.
Example 4 (nauseated-correct usage): The smell of rotten eggs makes me nauseated.
Example 3 illustrates the restrictive usage of the word nauseous, referring to the smell of rotten eggs, something which causes a feeling of sickness. In Example 4, nauseated, not nauseous, is used to refer to the actual feeling of sickness caused by the smell of the rotten eggs.
Please note that the word nauseating is synonymous with nauseous in that both refer to something that causes nausea. To say, “The smell of rotten eggs is nauseating” is the same as saying, “The smell of rotten eggs is nauseous.”