Headaches after races

I have raced 4 times total and 3 this year and have experienced a significant headache a couple hours after each one.   I am sure this is nutritional/hydration related but I am not sure what to change.  In the days leading up to the race, I always try to eat well and often.  And usually increase water intake for hydration. 

This past Saturday I had a race and  I had a a bagel and a bottle of water 2 hours before the race and sipped on Gatorade before the start.  Drank another bottle during the bike and drank 2-3 cups of water during the run.  After finishing, I had a couple small bottles of water and a 20 oz Powerade.  I tried sipping on a coke to try something different but it didnt go down well, so I only had 1/2 of it.  But I still had a headache later. 

Any suggestions?

Thanks in advance,
Mike