Actually Dan, I do think you were dehydrated.
You probably didn't drink anything from the time you woke up to the time you played football, right? If you did, it probably wasn't much.
If you weren't sweating while you were playing, that's really bad! It means you're dehydrated.
It sounds like you were really thirsty after you were done playing. You should be a little thirsty, but not that thirsty.
I run 5K races for cross country so I need to be very very well hydrated. If I'm not, bad stuff happens. Anyways, after the race I'm not even thirsty until about a half hour later. That's 5K, 3.1 miles, mind you! Not even thirsty.
It could also be caused by doing it in the morning, when you haven't eaten a lot. Your body is running on fumes, and you're asking for a blackout/headache!
So I think you were dehydrated, and you did too much too early in the morning, that's all.
My humble, quite educated guess