---Possible Dawnguard spoiler---
I have a question: I did not ever choose to become a vampire but for some reason when i finished the game everybody treats me like a Vampire, when i walk walk outside it says that my blood boils from the sun and i have red eyes even though i never choose to become one, i even have the option from Serena to become a Vampire??? When i entered Soul Cairn i choose to trap my soul so i don't understand? Anyone?
Now, whenever you contract the disease (i forget what it's called, but you can even get it from random vampire enemies) you will start becoming a Vampire if you don't heal yourself with a potion or at a temple after a few days. In the original game this just made people dislike you, but now with Dawnguard it adds a bunch of additional problems too.
Once your vampirism reaches an advanced stage, like where your blood boils and you can transform, you'll need more than just a potion to heal yourself. There's a whole ritual that needs to be performed as part of a quest. You gotta talk to someone (a dark elf "mage" looking character) in Dawnstar, I think.