I used my just built nether portal to leave the nether and my screen went black just before hearing a damage taking sound and instantly dying.

The death message said I suffocated on a wall after a couple seconds of a black screen and  it showed my stuff on the floor with the nether portal in a cave with no gravel and nothing that could suffocate me nor instantly kill me. Important to mention that this was on a realm of a friend of mine and I had full health before teleporting and the nether portal was just built a couple seconds ago. I lost my recently crafted and hard worked diamond armor and tools and also lost everything I had farmed for about three hours. Bummer...