Attend to Morthal, seek the hooded man... best of luck
You're sure you completed http://elderscrolls.wikia.com/wiki/Rising_At_Dawn? If so, then I have no idea why you're still a vampire. I guess you could try becoming a werewolf since that overrides vampirism and versa vice. Or reload a save from before you were infected.
Clearly, Molag Bal wants you. If you've done Rising at Dawn properly, and let Falion perform the ritual, your game might be glitched, although I've never heard this particular glitch before. Is there an older save you can load?
If your on PC, console commands should be able to fix it if none of the other options work.
The Daedra always win in the end...
are you on PC, or console? if PC you can always use the commands to rid yourself of it..
Amen!
PS...The word amen (/?ɑ??m?n/ or /?e??m?n/; Hebrew: ?????, Modern amen, Tiberian ?āmēn; Greek: ?μ?ν; Arabic: ?????, ?āmīn ; "So be it; truly") [1] is a declaration of affirmation[1].....[2] (Censored by Sah: contains religious contents} .....[3]Common English translations of the word amen include "verily" and "truly". It can also be used colloquially to express strong agreement, as in, for instance, amen to that.