That has happened to me a few times. Once was when I was drunk and I was smoking a ton and I held a hit in for a really long time and then my vision blacked out for a while, but it all came back once I blew out the hit. Another time I was driving with my friend smoking a joint and hot boxing and the same thing happened. I think it is probably just when you get really high past the point that you should and when you are holding in a hit too long that causes it.