First Brick...Dehydrated?

I've been training for my first tri which is these weekend in Hermosa Beach, CA and on Saturday I did my first "brick."  What I experienced afterwords is what's in question.

The brick consisted of a 20 mile bike (the last 4-5 of which are uphill) followed by a 3.5 mile run.  The bike ride took me 1:10 and the run took me 27 mins.  During the bike I consumed about 8 ounces of an electrolyte replacement drink and just prior to the run I had one gel.

I felt fine during the entire workout but I think I screwed up afterwards as I didn't think about re-hydrating myself.  About 2-3 hours later I started to feel really run-down and had a bad headache.  I realized that I hadn't properly replenished myself (although I did eat a healthy meal) and I tried to "catch up" by drinking fluids regularly throughout the day.  It didn't work and I felt bad almost all weekend.

Was this a case of not properly rehydrating myself after training or something else?