Methinks you were just running on adrenaline...or you managed to complete a REM cycle before you woke. REM cycles are approximately 90 min. If you wake up at the end (normally wake up after dreaming) you will feel refreshed and awake. If you wake up in the middle of one, you feel sleepy all day. Or it could be that you normally get too much sleep and if you oversleep, you can also feel "not awake"

Airmid, your friendly neighborhood Psychology student.