nausea is a side effect of Zoloft/Sertraline... perhaps 50 mg is too much for your system...
If you are just starting Zoloft/Sertraline, it may take some time for your system to get acclimated. All though this is doubtful, perhaps there is an ingredient that you may be allergic to... remember the half life is basically 24 hrs, so when you take another dose 24 hours after the previous dose you'll have 75 mg in your system on the second day, etc, etc.. If the Zoloft/Sertraline IS helping you, try taking half the 50 mg or 25 mg, then the following day 24hrs later, you'll have 25+12=37 mg vs 75 mg. and see if the nausea is less intense. good luck.