I was recently in the nether for the first time since the last update and i suddenly took constant (and fast-acting) fire damage but to my surprise there was no fire, lava or mobs (no source of heat) within an 8 block radius. Does the netherack conduct sufficient heat to apply burn damage?
No, netherrack alone cannot burn players.
You may have been hit by a ghast. If so, usually the ghast would blow up an area around you. Is this the case?
Another possibility is that you got hit by an arrow. If you are in multiplayer, an arrow with fire aspect could have been fired from another player.
Also, make sure it is fire damage, not hunger or fall damage.
Other than those points, this may just be a bug caused by lag or other reasons.