Lack of food, dehydration, heat...she should see a Dr though.
It does sound like your girlfriend was dehydrated, but it also sounds like she might have been having a seizure. My ex-husband was epileptic and found that misnaming a stable level of blood sugar was crucial to avoiding seizures. Please have her get checked out by a doctor.
I agree with the others that she most likely was dehydrated. I'm sure that was scary for you both. I would consider a check up just to be safe. good luck (and remember, hydrate, hydrate, hydrate).
I realize you're kids, but that's the kind of thing where I'd get medical attention for her right away. Passing out randomly isn't good, particularly in that kind of heat. Instead of just sitting on a bench, you should've taken her immediately to the guest services place where I'm sure there's a nurse or some other medical person who deals with people who get sick at the park. If it was dehydration or heat related, they could've taken care of that or told her if they thought it might be something worse.
The next time you're out in the hot sun all day, you need to make sure you're both drinking tons of water to stay hydrated. And if you're at six flags and she faints again, don't mess around and just get her medical attention immediately.
It sounds like the obvious explanation -- low blood sugar, heat and dehydration. (What was she thinking, not eating all day?) But it's worth her getting tested for diabetes just in case.