My back is going numb when I bend back.?

Okay so, that is a bit of an understatement. It's done this for years I just don't have any idea of how to explain it. When I bend back and hold my breath, after about 3-6 secs my entire body goes numb. I 'wake up' about 5 secs later. It is an unbelieveable experiance. It's like, getting really, really really high. so.. What's wrong with my back?