I get this way too if I don't eat or drink for awhile. I passed out LOTS when I was growing up. The most embarassing was when it happened in college in a chemistry lab, and 911 was called and I was taken away in an ambulance!
I was the talk of the class and being such a shy person, it was really hard for me to go back.
I make sure to always have my purse stocked with granola bars, or almonds, or something to snack on. I eat a little something between every meal too and it helps a lot. I don't get the faint feelings much at all anymore. It also happens to me if I'm standing up in one spot for too long, so I make sure if I'm at a meeting or something that I can always get there early enough to get a chair.
My doctor explained to me that I just have a very touchy system... if my blood sugar gets a little low or I get too hot or anything gets out of the "comfort zone", my body doesn't know what to do and just wants to shut down so my blood pressure drops and I pass out.