You haven't mentioned how many years you are married.
If you and your wife were used to sexual activity and if this vomiting is recent occurrence then she may be suffering from gastritis or may have developed a disgust for sex.
Next Steps
Usually people have sex at night and on a full stomach. Try and have sex on empty stomach and see if it solves your issues.
Try giving her antacids and anti emetic tablets before meals
have sex 3-4 hrs after meals
.
if vomits dont stop, seek a physician or psychiatrist.
Health Tips
Talking with her will give enough clue to make ammends