I was running through the game again after a lengthy absence from it. I reached the point where I first encounter Lord Harkon, and I refused his gift of vampirism. (I don't much care for being a vampire in this game - not the same for me as vampirism in Oblivion). When I get banished back outside, the screen goes red for a moment and I am informed that the sunlight boils my blood. Not a vampire, though. I thought maybe I could have contracted it earlier from another vampire and simply did not realize it (and it was merely an incredible coincidence that the symptoms started when I stepped out of Volkihir castle). I do not seem to have any actual ill-effects from being in direct sunlight, and I can still fast-travel during the day (something I thought one couldn't do as a vampire). I reached stage four (judging from the fact that Embrace of Shadows perk activated). Still no signs except for the flashing red screen and warnings about vampirism. At the entrance to the soul cairn, I could not enter the soul cairn. According to the UESP I should have been able to enter, even if I were only a normal vampire. I let Serana turn me into a vampire in order to enter, thinking that this would clear up the glitch. After the soul cairn, I sought out Falion in Morthal to cure my vampirism. He did. The symptoms seemed to stop until after I completed the Dawnguard questline. Now they are back. I have never been able to feed as a vampire in Skyrim, one of the reasons I don't like being one. None of the NPC's seem to notice that I have level 4 vampirism again, not even the sanguinophobic Isran. They chat me up as though I am normal. Anyone else had this?

If Serana (or any other NPC, but Serana is most likely) consumes a Potion of Blood, the script mistakenly assumes that the player is the one drinking it and tries to bring the player back to stage one vampirism. The end result is the player acquiring some vampire spells and the script being registered to update the player's vampire stage, without the player being considered a vampire by the game's logic.

I wonder if there is any way to reset this via the console.