Offhand I'll say there are only three ways to lose health ...
1) The most desireable way. You hit a Mysterious Shrine, adding 5 to one stat (usually Dexterity but not always) and taking 1 from each of the other three. Net gain of +2 to stats.
2) An occasionally desireable way. You hit a Fountain of Tears which just happened to select Vitality to take 1 from and add to another stat.
3) and what seems likely in your case and is to be avoided. You were hit by the zombie called Black Death which permanently subtracts a hit point. These spawn on levels 3, 4, and 5 and when present it is prudent to save frequently, after every kill of a Black Death or before opening a door, etc while keeping a constant eye on your hit points.
