Strange stone/water walls appearing in flat world

I created a flat world about one year ago and built a small town in it. I have had no problems with the world up until today, 7/21/17. Today I opened the world and went to the edge of the town. To my surprise the town was surrounded by huge walls of stone/ water. At the top of the walls are islands that look like a normal non flat, randomly generated world. Keep in mind the world I created was a flat one.