Invisibible

So far this is only happening in the Nether. For some reason, randomly and without warning, everything except the creatures and lava becomes see through. It's not like the blocks aren't there, I can bump into them. This isn't a huge issue for right now, because if I press esc and exit and save, than log back in, it resets. It seems to be happening more and more frequently, though. Pretty annoying.