Why does my girlfriend get nauseous when i ejaculate in her??

It's not the semen causing nausea. Something about the sexual encounter releases hormones and/or stimulates her vagus nerve which provokes nausea. Taking a medication like dicyclomine 1 hour before intercourse may prevent the nausea. Her gynecologist can advise her further.

This is unusual and without an explainable organic reason. Is she worried about something? you can try wearing a condom to see if that makes any difference. If there is an odor problem- then a condom and/ using another scent might help