Overheating and Lightheadedness: What's Going On?

For the past few months I've been having these little "episodes" where I overheat and get very lightheaded, weak, and shaky and it gets hard to breathe. I work at a movie theatre so it's very hot being surrounded by warmers, ovens, and a popcorn popper for hours. Last night I got to the point that I nearly passed out and almost fell over and I had to be driven home. Usually sitting down for a bit helps me feel better, but last night it didn't go away until I was driven home and rested overnight. I'm just wondering if anyone has any idea of what's going on? I'm always hydrated and I eat enough so I don't have any idea.