I hear it all the time--even on television: "I'm nauseous."
Tsk, tsk. Are you really?
Nauseous is an adjective that means causing nausea; sickening, disgusting; sickening to contemplate.
Nauseated is a verb that means to feel nausea, become sick; sick at the stomach.
So when you say, "I'm nauseous," you're actually saying, "I'm sickening, disgusting, and sickening to think about. I cause nausea in others."
“Do not, therefore, say ‘I feel nauseous’ unless you are sure you have that effect on others.” (Strunk and White in The Elements of Style, p. 53)
If you're feeling like you want to throw up, you're nauseated. If folks can't stand to be around you, you're nauseous.
Know the difference!