Why did i blackout and feel nauseous after anal sex?

Vasovagal syncope?: I would guess you had vasovagal syncope (pronounced vay-zo-vay'-gul sink'-o-pee). That's medical talk for a standard fainting spell. Usually fainting is triggered by a stressful experience, either emotional or physical, e.g. having blood drawn. Anal sex could do it, especially if this was a new or uncommon experience for you.