Huge chunk border suddenly appeared today close to my newbie house.

After 3 months on Minecraft single player, a huge chunk border suddenly appeared close to my newbie house where I still live.This huge four-cornered thing is in the view I see every time I leave my house now. Why? Can I get rid of it? I built a new house across the way but this big monster can also be seen the second I leave my new house as well. If I use TNT to rid myself of it, will it reappear? I don't know yet if this is the only change in my world. It is MY world isn't it?