I pass out too, as do my daughter and my brother. It is because we are what is called blood-injury phobics. I can't handle blood, gore or the thought of anything cutting or piercing the flesh.
I once passed out in the cinema, many years ago when watching The Exorcist; it wasn't the horror stuff, that doesn't tend to bother me, it was right near the beginning of the film, when the girl had behaved slightly oddly and was in a hospital for tests. She was lying down, there was some tube attached to her, which is what made me very edgy, and a machine going chugga chugga... this medical scene made me panic and I got overwhelmed with adrenaline, due to my phobia, and passed out. I didn't see the rest of the film