After upgrading to 1.1 my main world opened on a 3 weeks old save

I just upgraded to minecraft 1.1 (previous was 1.09). Opened my main world (survival, normal) where I worked in the ender for days to find shulkers (to get these shulker storages for the update), and I constructed some restone stuff around my house. I was back in my house in the last save yesterday, 100% sure. But when I opened it today after updating I was in the Nether, with random fortress loot in my inventory. I soon realized that this was my game status about 2-3 weeks ago, probably 15-20 hours of game play. Went back home, found my house indeed in the status it was 3 weeks ago (no restone construction, lost all shulkers I have collected ...).

In short, I opened my world on a save that dates back from 3 weeks / 15-20 hours gameplay ago.

I dont know if this is 1.1 or another random bug, but definitely annoying. I guess there is no way to recover these last 3 weeks work on this world?

Thanks