I'm going to guess it's hyponatremia. Riding in the 40s F, I'll drink about one liter in 30 miles, if that, and have a glass of recovery drink upon finishing. Some of my buddies wouldn't drink anything during a ride like that. So I'm saying you drank too much. Easy to test. Next time you go out for a ride of this length, try drinking just the one bottle, or drink to thirst: if you get thirsty, drink a few swallows. Never drink unless you are thirsty. That's the new rule. "Drink before you're thirsty" got a lot of people into trouble. I'm thinking that "drinking lots of water" after you got back is what actually brought on the headache.
There are other possibilities. It may be low blood sugar. Next time, drink a pint of chocolate milk as soon as you get back, then nap. It may also be exercise-induced anaphylaxis, but that seems unlikely considering what you ate.