If it isn't acid reflux or allergies you might also consider migraines. Nausea is one of my symptoms, and fairly common in children too, as I understand it.
She's just old enough that hormones could be playing into their onset, depending probably on how developed she is.
I know I lived with them for several years before they were diagnosed. I am able to negate them with diet, extra sleep, vitamins & chiropractic. When all that fails simple OTC Aleeve seems to work best. It's different for everyone though.
It's a common misconception that migraines are always painful or that you need a dark room and quiet to get thru them. You can have visual "light shows" (bad dream?), feel like you're spinning, throw up... the list goes on.
And, if she doesn't have any other symptoms, the effort of throwing up may cause her body to create enough adrenaline to stop the rest of the migraine from developing!
Talk to her Doctor and keep investigating... you'll either figure it out, or she'll grow out of it. Just try not to freak her out worrying about it! Sometimes our bodies (especially kid's) do weird things.
I hope you find a solution and that she feels 100% soon!
Oh, and Rotavirus is accompanied by a very pungent smelling diarreah and fever. If you've ever had it in your household, you'll know the smell! *ick*