Fainting during and after sex?!?!

I've completely passed out 4 times in the last few months from having sex. It only happens if I'm doing it doggy style though. Sometimes it happens during sex and sometimes it happens after. I get extremely nauseous really fast then I pass out for 30 seconds to 2 minutes and wake up incoherent and not remembering what just happened. There's no lingering side effects or symptoms once is all said and done either besides becoming so incredibly tired I can barely keep my eyes open. Is this just a thing or should I see my doctor? If I should see a doctor, should it be my gyno or my primary care physician? Thank you!!