So, I'm not sure what happened here, but I assume one of the steps here did something to make it happen.
I had 2 HP and an Adventurer's Vest.
I picked up a Pink Shirt, equipped it, still 2 HP.
I went into the next room, the host dies, I get hit. My HP is graphically 0. Room cleared, 0 HP, strange.
We get a Bishop who offers to make enemies deal double damage. Well, he's at 1 HP and I'm at 0 HP, sooo... what's the harm? Accepted.
Next room, I go in and kill everything, my HP jumps to 18. Then a health orb appears on the ground from the Bishop, I jump to 120-160 something HP.
Where did that sudden boost of 18 HP come from, and why wasn't I dead? I'm 90% sure I didn't level up.