I've had similar experience. Had a dream about my dead grandpa (at some point I realized he's dead and he turned into a zombie along with other people around - not very nice dream). Then I dreamt that I've woken up, I've seen the dormitory in which I really slept, went to open door cause somebody wa ringing, and it was the zombies :/, at which point I've woke up again, for real this time. My friend from the room said I've sat up on the bed and "looked at him" with closed eyes for like 5 minutes, before waking up.

It might have sth to do with drinking like 2 liters of strong tea and studying for exam whole night and day before.