With h1n1 it might be even harder to tell if Tamiflu is behind any nausea or vomiting that you see, since unlike what you usually see with flu, h1n1 sometimes comes with vomiting and/or diarrhea.
I can't remember if any stomach upset came with mine- I was so sick that first day I was barely awake once my mom got me home from the doctor and all I wanted to do was sleep. I know I didn't actually throw up, though.