I usually sleep for only 4-5 hours. Thats how its been with me for the past year or so. Sometimes i really feel like sleeping in and then i go all the way to 12-14 hours. I did that last night and now theres a nuclear war going on in my head. Why can i not enjoy sleep when i have the chance to? 
^ur head isn't used to getting so much sleep......so the headache is actually due to long sleeping