Dawnguard DLC Vampire bug

[Bwammoz]

I got the Dawnguard DLC, and I joined Harkon with Clan Volikhar I think it was called. After a while I saw somewhere that you can be a blood-starved vampire without anyone attacking on sight, so I waited until I got it and... Everyone attacked me. Then I went back to Castle Volikhar to feed on those thralls, and everyone there attacked me too... After that I went to Septimus Signus, used Vampire Seduce or whatever it is called, to feed, and went back to the castle, and everything was normal... Can someone please explain why this happened?

[wowszer]

Well, normally when you join with Harkon he bites you, you wake up to him giving you instructions in your new abilities as a vampire lord. If you declined him he will basically say get out, and you finish on the side of the Dawnguard. In this option you can get the regular vampire sickness, but not in the first option. You can't become a regular vampire if you are already a Vampire Lord. Unless you follow up on the rumor of a vampire cure and have it performed. But you would no longer be a Vampire Lord. Have you any Mods, some mods or mod orders can have weird effects.

[Bwammoz]

I still want to be a vampire lord, I've completed the questline on another character through the Dawnguard side.