Why are vampires at Castle Volkihar attacking me even though I'm a Vampire Lord, too?
Solution 1:
Found solution.
You must teleport to another location with console, spell or just running away from the castle, and then return back. So this problem will be fixed.
Solution 2:
They are not meant to attack you on your first visit to the castle or at all if you are following the Vampire Lord questline. Turning into a vampire lord will never pacify a hostile NPC, and your choice of words leads me to believe they were hostile to you before your transformation. Your best bet is to reload to a save before aggravating them.