It may be due to penis touching the cervix and if the ovaries are in a position in which they can be touched by the pressure in the vagina, and these can produce nausea or light-headedness. Even orgasm can produce nausea in some. Try having intercourse 2 -3 hours after meals and avoid intercourse immediately after meals.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.