I had a semi-black out from the sun heat?

A few years ago, I was in Sea World waiting in line for a rollercoaster. It was intensely hot and I was surrounded by a lot of people. Anyways, for about 5 minutes, I started losing my eyesight, like, everything started fading out to black until all I could see was pitch black. I could barely hear anything and I had to be taken out of the line cause I almost fell. It was sort of how I felt before the time I fainted from donating blood. Anyways, what was it, and what could have caused it?