Board Thread:Skyrim/@comment-113.210.2.3-20150731083911/@comment-6881629-20150731202714

Some enemy grabbing her attention that got missed on the way there maybe? Probably bad advise but follow her and see where she goes she may eventually stop, or keep going without her and see if she appears when you exit at the end of the dungeon (or fast travel after). Serana's very glitchy though so keep a save before trying anything, if you have a save before starting the quest maybe go back and try it again.