[MattR]

It sounds like you are taking in plenty of water based on your description. Perhaps you are drinking too much and swelling your brain.

I get headaches after rides too. Mine are typically caused by some dehydration and/or neck strain. So I drink plenty like you do to help minimize the chance, but I think the neck strain may be a bigger factor. With the weight of your head and helmet, you probably have about 15-20 lbs bobbing around on your neck. This can cause neck strain during longer rides. I like to take a Tylenol when I get home and this eases the tension and reduces any inflamation.