This has happened several times to me but when I am building straight up In the nether and do it for more that a few blocks in a row I will take damage. Usually it is enough to instantly kill me when I'm at full hearts, although there have been a few exceptions like it takes me down really low or the one time when I caught on fire. I think that the game thinks that when you are building it thinks you're falling so when you stop for a moment it thinks you hit the ground so it causes fall damage (Usually enough to kill) but I'm not sure about the time that I also caught fire.