Sounds like you might have had the beginning stages of hyponatremia. This occurs when people over-hydrate themselves (happens to marathoners a lot). It's an electrolyte imbalance resulting from the inability of your kidneys from excreting so much excess fluid. Essentially, your body doesn't know what to do with so much excess fluid at once.
Regular ol' 8 oz servings of fluids every half hour or so would probably be sufficient for next time! I'm glad it worked out okay!