For a week, I have been experiencing on and off (overwhelming) nausea--even Zofran (ondansetron) doesn't help. No vomiting/diarrhea so I'm starting worry.

Nausea. Before you go and worry yourself to death just take a deep breath. Zofran doesn't work for everyone..especially if you can't keep it down. Consider a suppository of Compazine, or even better my favorite Phenergan (promethazine). It works well and you don't have to worry about throwing it back up. Go see your primary as well. Routine blood testing may help. Don't forget pregnancy testing.