Night Headache

After a 2.5 hour workout that included swimming I got a headache tonight. Came on rather quickly. Tried to eat/hydrate but it kept coming so I took 1000mg motrin. Woke up and it was still fluttering around but generally gone. High correlation to swimming. I may be dehydrating myself more than I realize on these workouts. I came home and drank three or four bottles of water without peeing.