I noticed that sometimes I'll get randomly hit by an unexisting entity. I even checked with hitboxes and there's nothing.
Now, whenever I get close to the blood stain, it'll damage me, as if it was a hostile mob.
The first attack though, I have no idea why it damages me.

I was just killed by it and it says that it was a Zombie Crawler, although I didn't see any hitboxes or anything, as if he wasn't there.