My English doesn't perfect so I will tell simply. I completed main quest, Alduin gone to hell, Skyrim is saved, everybody happy. I'm playing another quests. 10 minutes ago I gone to Throat of the World. I want to fall down (for find my quest location) so I jump down but sudennly I heard weird sounds and look to up to Throat of the Wolrd. I saw storm and fireballs have falling on to me. I waited few seconds and fast traveled to Throat of the Wolrd and suprise! Alduin's back. I tried to kill him but he didn't die. What's going on? I didn't use cheat.
Here the video http://youtu.be/3OwX11ztE18 Talos bless auto-save