Teleported into sand

I was playng on my hardcore world chopping down trees when i opened my inventory to sort out useless items for saplings on the ground, but suddently everything besides my inventory went black. I closed my inventory and assumed i was inside a newly-grown tree so i switched to my axe, but when i started mining it i heard sand so i switched to my shovel. I started to panic, because i was dying. I rejoined, and i was still there. I ate some dried kelp but soon started taking damage again. And i died. When i spectated my world i could see a 2 block wide, 2 block deep hole, with my stuff spread on the ground. I don't know how deep down i was, as sand is a gravity block.