Oblivion!

you are a vampire because you have been infected with vampirism, after you sleep you develop it and unless you feed off of the blood of other you develop more serious side effects as well as benefits. you are dieing from sun damage.

there is a quest you can do to find a cure but it ain't always easy. the game won't change too much but it will make travelling harder.

omg am doomed!, i hate vampires!, this is not cool, so i have to feed on people once nd den?

he's experienced in this (and not just in oblivion XD)

You need to sneak on people in bed to drink blood in order to stay in the sun. If the don't drink blood you get hurt by the sun but you get special powers. And you'll look weird and people will notice that you look weird.

Moral of the story: make plenty of saves. You'll never know if you'll need to use them. Didn't you pay attention to the load-screen hints that mentioned "Pophyrric Hemophilia?" That should've tipped you off before it was too late.

But as for now, it's as the others above have mentioned.

BTW you can get rid of it right away if you chug a cure disease potion right after you get it and before you try to sleep. This don't work if you already slept with the disease.