Hah, that happened to me once. One time I woke up at 1 PM when I slept early the day before. Heh, that was a weird moment for me. It screwed up breakfast and nearly ruined lunch.
I remember asking Falconstar if he thought my headache was a migraine or just a bad headache. I asked him because I knew that he had one before (he told me). There wasn't really a way that I could have known because I hadn't had one before. But I'm pretty positive it was more than just a headache because it lasted around 12 hours before I finally decided that I couldn't take it anymore and went to bed.