Hello! I'm sorry to give you perhaps bad news.
This is the link to the vampire section in the wiki:
http://uesp.net/wiki/Oblivion:Vampirism
It sounds like you had a disease that sapped your strength. If you click the spell icon on your player menu, to the right on the next screen is an icon that gives you a menu of all active spells your character has, to include diseases. It's always a good idea to check this after any combat. If you do this now, you can see if your character still has vampirism. Good luck!