Ok, I started playing this game over a year ago and don't really remember the specifics of what happened.  I started playing it again, and for some reason, I turned into a god damn vampire. It seems I have to feed on people while they are sleeping, or else I die when I go outside during the day time.
 
Other than that dark brotherhood guy, I didn't even know vampires were a "thing" in this game.  What the fuck is going on? 
 

Welcome to one of the most frustrating things about that game.  While Vampirism comes with a few ups, they're drastically outweighed by the downs (not being able to do anything in the day being the biggest).  The quest to undo the Vampirism is a pain in the ass.  Good luck, duder.

Also you can get the Vampire sickness by fighting them.  You would've gotten a notice that you feel ill and you would've seen that you were poisoned.

 If you didn't chose it, you must have gotten it while fighting a vampire.  There is someone with a dialogue option Cure for Vampirism.

are you playing this on pc? if it's pc then you can just use console commands to fix it.
if I were to try and think of them... they'd probably resemble something like:
set vampire to -1
player.setfactionrank playervampire -1
 
and then you'd have to go through all of the vampirism spells, removing them by typing:  
player.removespell (vampire spell here)
 
and you may have to remove your vampire stats as well.
again, this would be done with the removespell command.

" Also you can get the Vampire sickness by fighting them.  You would've gotten a notice that you feel ill and you would've seen that you were poisoned. "

There was DLC that cost about $3 that had the ability to de-vampireize (Now a real word) yourself.
Since I was lazy and had $3 lying around, I bought it.

" @MAN_FLANNEL:  
How to cure vampirism in Oblivion.  "

" @Mordukai said:

" @MAN_FLANNEL:  
How to cure vampirism in Oblivion.  "

Um, I still think I'm not a "full" vampire.  I drank some dudes blood and I can go outside in the daytime.  That website said something about praying in a church...does that work? "

you get it from fighting them but it is called something weird. if you dont cure it in a day it turns you into a vampire

You have four options:
 
- Do longass normal quest to cure vampirism
- Purchase Vile Lair DLC (150 MSP) and use the Font of Renewal located within (mine some vampirism-curing powder stuff off the nearby rocks)
- Feed on people daily to avoid the bad effects of vampirism
- Reload a pre-vampirism save

I stopped playing after I turned into a Vampire. 20 hours in and now I can only go out at certain times of the day. Lame.

" I stopped playing after I turned into a Vampire. 20 hours in and now I can only go out at certain times of the day. Lame. "

" @Mordukai said:

" @MAN_FLANNEL:  
How to cure vampirism in Oblivion.  "

Um, I still think I'm not a "full" vampire.  I drank some dudes blood and I can go outside in the daytime.  That website said something about praying in a church...does that work? "

Eh, must have gotten bitten before I stopped playing, because when I started playing tonight I was still normal.  If I just have to feed on people then I guess it isn't too bad.
 
I think I'm getting towards the end of the main questline...letting the gates in Bruma stay open to get some stone...how far away am I?

I turned into a vampire without realizing it as well. Once I saw the quest required an inordinate amount of work to complete, I just decided to say "fuck it" and reloaded an old save. Luckily I had only lost about 3 hours of game time which is small potatoes in how long that game takes to complete and how long it probably wouldve taken to do that stupid cure quest anyway. And to hell with paying money just to solve an affliction like that. Almost as bad as horse armor.

" I stopped playing after I turned into a Vampire. 20 hours in and now I can only go out at certain times of the day. Lame. "

" You have four options:
 
- Do longass normal quest to cure vampirism
- Purchase Vile Lair DLC (150 MSP) and use the Font of Renewal located within (mine some vampirism-curing powder stuff off the nearby rocks) - Feed on people daily to avoid the bad effects of vampirism- Reload a pre-vampirism save "

do you sparkle?

" do you sparkle? "

That has got to be one of the most punishing gameplay ramifications in any video game.

I dont even sparkle

I loved playing as a vampire. Going out in the sun doesn't hurt that bad, and I never drank anyone's blood. You get some great buffs if you don't feed.

Don't make the mistake of feeding on Imperial guardsmen.  Not only do those assholes have psychic powers, they'll wake up and attempt to arrest you if you feed on them.

" I loved playing as a vampire. Going out in the sun doesn't hurt that bad, and I never drank anyone's blood. You get some great buffs if you don't feed. "

" @MAN_FLANNEL: You're pretty close to the end of the main quest. After closing the Great Gate, you do one long task, then one short task, and it's over. "