Headaches after I ride

[stay_upright]

I was riding sat and sun this w/e. Saturday about 2 hours after riding at home I get a headache which goes away later in the evening. I drank about 2 litres of water while riding. Sunday I make a note to drink a lot and drank about 3 litres of water plus 1 pint when i got home - no headache ... I had 1 pint of beer later on. This morning I've got a headache more or less from when I woke and continued all morning. I drank I pint of water - not much improvement (if any). When riding I was trying to drink more water than my thirst would ask for i.e. not waiting until I was thirsty - any suggestions? is it dehydration? I'm wondering if it could also be lack of salts or something? I just drink water when I'm out.