Summary
As usual I was playing Minecraft.
Then suddenly someone attacked me.
However, I looked around and saw no mobs.
The attack caused a knockback and the attack power is probably about 2HP.
Also when the attack occurs I hear a high pitched painful voice.

Situation
There are no hostile mobs around that can attack from a distance, such as phantoms or skeletons.
Without the dreaded bug situation where falling damage occurs even though you are not falling.
Lag is not possible.
I tried it without the texture pack but the result is the same.
I am not wearing the thorn armor.
Cheats are off.
It happens in both single and multiple players.
These attacks happen suddenly and repeatedly.

Thank you.