After I destroy the harvest hear in oblivion by helping the vampire mother, I do not get any other AD quests. Is it because i'm level 46? I can't seem to find the quest anywhere
Someone (possibly Gathwen?) should have run up to you when you finished the quest and added three markers to your map. One of those was with reference to Vanus Galerion and King Dynar, who are waiting for you just to the north, at the Reaver Citadel. That's where the zone questline goes next.
(The other two markers are for side quests which add to your allies for the upcoming battle.)