Okay, I understand that you can be infected by a vampire during combat and not notice that you have contracted Sanguinare Vampiris until it's too late. I get that.

But what if you're ambling along, enjoying the bright sunny day, shooting wolves and bandits and generally having a grand ole time, and suddenly the screen flares yellow and you get a message "your vampire blood boils in the sunlight"? This has been happening to me recently (the past couple of days), and I'm not finding any explanation.

A reload of a save prior to this bizarre Sudden Vampire Syndrome (SVS, I call it) shows no signs of disease or vampiric powers; nothing to indicate that moments later, my blood, suddenly vampiric, will be boiling in the sunlight. No vampire powers, no messages about strange hungers as the sun sets.

Some specifics: I've got the Dawnguard DLC, and have completed both the Dawnguard faction questline, and the Vampire Lord questline (as different characters). I'm now running a third character through the Dawnguard faction questline because I didn't get the Enhanced Crossbow the first time, and wanted to see how well it performed. The first time SVS happened to me was yesterday, while I was in Fort Dawnguard. I suddenly get the message that my vampire blood is boiling in the sunlight—indoors.

Today, using an earlier save and carefully confirming that I had no signs of disease (and no signs of vampiric powers), then drinking a Cure Disease potion and getting blessed at a shrine for good measure, I (and my follower, the vampire Serana) storm Faldar's Tooth in search of a woodcutter's axe (I sold the few I found earlier, and now I need one to cut firewood for arrows; amazing how hard a woodcutter's axe is to find when you need one). I fast-travel to the Riften Stables, and walk in broad daylight to Faldar's Tooth. While I am fighting the bandits pouring out of the fort, my screen flares yellow and my "vampire blood" begins its sunlight boil. Huh?

Again, a check of Active Effects prior to the occurrence of SVS shows nothing in the red; no Sanguinare Vampiris, no weakness to fire. Nothing. Not even girl cooties.

Anyone else encounter this? Am I missing something?

Update: Serana may be the cause of this. It appears that it's possible to get caught in her Vampiric Drain spell if you're between her and the enemy, but why this would instantly turn you into a vampire (requiring multiple quests to cure), rather than give you potion/blessing-curable Sanguinare Vampiris seems problematic. BTW: this does NOT turn you into a Vampire Lord (which Serana can do upon request under certain circumstances).

Update II: Serana is definitely the cause of this, and it almost certainly is a bug/glitch (I use the Xbox 360); I completed the entire Dawnguard questline (as a nonvampire member of the Dawnguard) with Serana as a follower and never had a problem with her Vampiric Drain during combat. This time, however, it is a constant problem; before I save, I have to check to make sure she hasn't winged me with that spell.

Update III: So I decided instead of constantly reloading to escape the SVS, I'd just go with it in hopes that I could go get it cured with Falion. But I discovered an interesting thing. I am currently a Stage Four Vampire (except I only have 50% resistance to frost, and 50% weakness to fire--but I DO have Embrace of Shadows and all the other vampire powers of the fourth stage), which means all humans should be openly hostile to me. But they're not. Which would be nice, except that no bartenders even subconsiciously recognize me as a vampire and tell me of Falion, and Falion himself doesn't offer me the option to cure vampirism or even start a conversation about it. Also, despite extreme vampiric hunger (I haven't had blood, ever, and I've waited 10 days to get to stage four faster), I cannot feed on sleeping humans, or any other humans, for that matter. I've considered joining the Companions and slogging through to become a werewolf, which SHOULD cure me of vampirism, but to tell the truth, I don't want to. Instead, I'm just gonna start a new character and try the Dawnguard faction questline again.