Dizziness and temporary blindness?

I sometimes get dizzy or my vision turns black to the point where I can't see for 5 seconds to 1 minute after I stand up after being stationary for a long while. This happens after spending time on a computer, watching a movie or even after 1 hour cles at school. I find it hard to concentrate on what's going on around me or what people are talking to me because of it. Any idea what this is or how it could be treated? My friend thinks I could be diabetic and after reading up on symptoms and causes it seems like I could possibly have it.