The adjectives, nauseated and nauseous, are very close in meaning. Both describe the feeling of being sick to one’s stomach—the feeling of needing to vomit (“throw up”). The only difference is that nauseous simply describes the feeling, while nauseated is the past tense or past participle of the verb to nauseate, which means “to cause one to feel nauseous” and therefore nauseated implies a cause:
She felt nauseous/*nauseated for no reason.
She felt *nauseous/nauseated by the politician’s selfish lies.
In many cases both words work equally well:
She felt nauseated/nauseous after watching the news.
Most of the time, people use nauseous for the adjective, and nauseated as a verb:
The former president nauseates me/makes me feel nauseous.