What Causes Excessive Drowsiness With Headache?

For the last 2 days i have felt excessively sleepy, at all times of the day. Even after sleeping for 13 straight hours i felt exhausted. I have also started to feel sick, and have a really painful headache. But its the excessive drowsiness that is most alarming to me, as i'm never usually that tired. Any ideas of what is causing this sudden sleepiness?