I was fighting with an Iron Sword a Blaze and one of its fireballs hit me. For some reason, after the fireball hit me, my hotbar displayed Absorption-like hearts, and after another hit of the blaze, with about 8 hearts plus the absorption one, I died.

The Realm wasn't using resource packs or behavior packs and neither was I. It is important to mention that all of this happened in the Nether and the world wasn't generated in 1.4.4 (in was generated in a previous version, unknown to me).