Something that turns your stomach makes you feel nauseated, and the thing itself is nauseous. Thus, something that causes nausea is nauseous; so that deviled ham and onion sandwich you ate for lunch is nauseous, but feeling that you would like to kneel in front of the toilet means that you are nauseated.

Remember the distinction. You should never say, "I feel nauseous," unless you — personally — are making other people ill.