Once you enter a nether portal,theres a chance you can take damage through suffocation.
This usually occurs when nether portals are spawned in tight locations and once you enter you suffocate for a secind and take damage,although i dont lose hearts on my experience...
AND I SWEAR TO GOD IF SOMEONE ALREADY REPORTED THIS,I SWEAR LOOKED IT UP AND FOUND NOTHING ABOUT THIS SUFFOCATION GLITCH.

P.S.-Its not because of the blocks... Its just when you enter ONE portal BLOCK and enter a tight area.