Skyrim Dawnguard problem

Solution 1:

In the Dawnguard Questline, the most common enemy type encountered in the beginning are vampires. Taking damage from a vampire's health drain spell has a chance to inflict Sanguinare Vampiris, a disease that will eventually progress to vampirism.

What you are experiencing is vanilla Skyrim vampirism caused by the progression of Sanguinare Vampiris. You may have contracted this disease in your 'everyday' dungeon clearing as this is a vanilla feature.

If you do not wish to reload your save, but wish to cure your vampirism, there are several ways to do so, as detailed on the Wiki page for vampirism. These include finishing the quest Rising at Dawn, becoming a werewolf, or console commands.