I am guilty of incorrect grammar. “I’m feeling so nauseous,” is incorrect. Grammar books and my sister stipulate that you must say, “I’m feeling nauseated.” In one sense, who cares — you’re sick.
For some reason, that sounds so stuffy — “I say, I’m feeling nauseated, old chap, hand me that trash receptacle over there.”
You are only supposed to use it as in “the smell of garbage is nauseous.”  That sounds funny to me. I would say, “the smell of garbage makes me nauseated.” (which is also fine)
In fact, the word nauseous is often heard as describing a feeling of sickness (instead of nauseated). And because it is commonly used that way, it may become acceptable. They may have to change the dictionaries and grammar books.
Funny thing, but the Oxford English Dictionary (OED) that gives definitions and usage of words, notes the first usage of nauseous in 1885–
“nauseous:” Of a person: affected with nausea; having an unsettled stomach; (fig.) disgusted, affected with distaste or loathing.
So, it is perfectly acceptable to say, “I feel nauseous.”   

That was the original usage.

photo: Doris Day in “Teacher’s Pet.”