Why do i feel nauseas after sex?

Ive only been able to find answers to this question as they pertain to women. I am a 22 year old male. I've been having sex for 6 years. Im not on any medications and don't drink or do drugs. I've been having sex with my girlfriend for 3 months now and everytime, about 15 minutes after we finish, I start feeling nauseous. Its almost like a hung over feeling (groggy and foggy and queezy). Any ideas?