So I started the mission Daedra's Bestfriend. I am on my way to get the rueful axe when I saw a goat on the road, I killed it with Fus-Ro-Dah when suddenly a vampire NPC run towards me, I thought she will attack me but when I sheathe my bow she did the same to her dagger and she wont stop following and doing what I am doing... When I talk to her it just say "This person is busy".
I haven't started the dawnguard DLC mission yet.
I am curious what did I do to trigger this. Or is it just a random encounter?
Edit: I can now talk to her and command her like a companion after reloading a save file.