All of that said -- the nausea could certainly be attributed to stress -- but as a physician my first instinct would be due to the blunt force trauma Zimmerman experienced the previous day. Most honest and competent medical professionals would agree with such a diagnosis.
If you have nausea hours or the next day after a blow to the head you should always seek medical attention -- that said,.. you can definitely experience nausea due to the increased pressure within the cranial cavity and end up fine as the pressures lowers itself over time.
Lastly, I haven't been watching this trial -- so I am unsure if this medical opinion was from the prosecutor's side or the defense. If it was an "expert" representing the state then they said just what they were supposed to , to bolster the state's case. If from the defense, then it was a mistake.
Further, it cannot be ruled out that this was simply a bit of incompetence or merely a lack of being thorough.
An honest answer would have been that the nausea could have been attributed to the blow to the head causing ICP, or simply a psychogenic source, such as stress.
Take care,