I spawned in a new world, and was jittering up and down very quickly. This does happen, so I flew up a bit, then moved over to a bit of flat ground next to a swamp. I tried to fly down, but an invisible floor was stopping me. I moved over to another area close by and I could fly and jump down. I then walked over to the original area, tried to fly up and got stopped by the same invisible barrier. It was only over one bit of land; suggestions?