Hi Welcome to HCM I think most of the time nausea after sex is unrelated to it. It might be your subjective feeling. Nausea may be due to stomach upset, which might be exaggerated during sex as result of pressure on the stomach. I advice you to speak with your girl friend, whether she has any stomach upset or reflux disease. Treatment will be over the counter available medicine like antacid, h2 blockers and ppi s will be helpful. Rarely the nauseas feeling after sex is usually caused by either a hormonal imbalance, or from the cervix being hit continuously during sex. It is unlikely as you have not mentioned any previous incidents. I advice you to treat the stomach upset if it is there. The problem should resolve on its own in week or so. I hope i have provided the information you needed. Take care.