So so weird, So heres what happened

I was Making a Exp Grinder and underneath the block where im standing is a Cave, So i distroyed a block in front of me to look down and see whats Going on inside that cave, im still above bcs i dont have a plan to go down there i just want to look, and when i look down i taken a half heart, no one or nothing is behind me but It takes damage. It doesnt happen like everytime i look down, but sometimes i look down on a 4-7blocks deep area i take damage.

-my boots has Feather falling IV and im holding a riptide 2 trident