My girlfriend gets nauseas and a headache after sex every time. what is causing this?

Headache with sex: Could be a migraine or a bleed so get her examined

Probably not you!: If she is having an orgasm, her blood pressure will go up for a few minutes. If she happens to have a blood vessel in the brain that is weak, this blood pressure elevation could be dangerous to her and indicate that the vessel is overstressed or threatening to leak. She should be checked to see whether this is a possibility, and meanwhile, sorry, you should not try for orgasm.

Possible Sexual HA: Although the details in your question are limited, what you describe is consistent with a rare form of a headache disorder called Sexual Headaches or Coital Cephalalgia. She should be evaluated by a practitioner who specializes in headache disorders to rule out other serious conditions and to offer appropriate treatment for her complaints.

Sexual headache: These are usually benign headaches that cause no further complications. About 1% of the people get them with orgasm, but this is possibly an underestimate. Most believe it is a variant of migraine, other believe it occurs from muscle tension during orgasm. An eval is indicated to make sure nothing else is going on, there are clear dx criteria which with a little more info can also help dx this.