Oh no, that’s no fun @Deborah813! Nausea is the worst feeling 
It could be that you are focusing so intently that the muscles in your body and your mind become overly tense. While focus is important, too much tension in your mind and body can lead to negative effects- such as nausea or even body aches/pains post session!
To avoid any icky feelings during spell casting, maybe consider casting in quiet and familiar places where there are less distractions and you feel you can truly relax without focusing too hard 
If you feel the nausea begin to rise: open your eyes, hold out your hands palms up, and begin breathing deeply from the depths of your stomach. Visualize this action as your spell charging from natural energy, while you give your mental focus a breather 
I also personally recommend ginger 
. Taken as a tea, herbal supplement, candies, or added to meals- it helps combat nausea.
Good luck- casting can be draining at times, but spellwork and meditating should be good for your body and mind. Nausea be gone! 